Patent Publication Number: US-8973812-B2

Title: Cluster box mail delivery unit having security features

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation and claims the priority benefit of U.S. patent application Ser. No. 11/168,471 filed Jun. 29, 2005, now U.S. Pat. No. 8,661,862 which claims the priority benefit of U.S. provisional application 60/583,419 filed Jun. 29, 2004, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to mail and parcel delivery receptacles of the general type referred to by the U.S. Postal Service (USPS) as “cluster box units.” More particularly, a cluster box unit, or “CBU” as it often is called, typically takes the form of a free-standing structure having a protective outer cabinet or enclosure, the interior of which can be accessed by USPS personnel by unlocking and opening one or a pair of so-called “master loading doors” that define a majority of the front face of a CBU. 
     2. Description of the Related Art 
     In a cluster box unit of the type having a left master loading door (LMLD) and a right master loading door (RMLD), the left side of the left master loading door usually is coupled by a left hinge to the left side of the enclosure of the CBU, and the right side of the right master loading door usually is coupled by a right hinge to the right side of the enclosure of the CBU. When the LMLD and RMLD are unlocked and pivoted to their fully open positions, substantially unobstructed access is provided to the interior of the CBU so that USPS personnel can insert mail and parcels into delivered mail and delivered parcel compartments arranged in left and right “stacks.” When the master loading doors are pivoted to their closed positions and locked, access to individual ones of the delivered mail and parcel compartments is gained by unlocking individual mail and parcel compartment doors that comprise door-within-a-door elements of the master loading doors. 
     Stated in another way, each delivered mail compartment and each delivered parcel compartment has its own, individual door, and these individual doors are elements of the left and right master loading doors—elements that normally are locked in closed positions so they pivot together with other elements of the master loading doors between closed and open positions of the master loading doors. Access to individual delivered mail compartments can be had by the customers, tenants or so-called “postal patrons” to whom the delivered mail compartments have been assigned when the postal patrons insert and turn individually assigned keys (that have been provided to them by the USPS or by managerial staff of apartment complexes, condominiums and the like) into locks found on the doors of delivered mail compartments so the doors can be opened to permit removal of compartment contents, thereafter the doors are closed and relocked, and the keys thereto removed by the postal patrons. A postal patron opens one of the delivered parcel compartments only when he or she finds a key to a delivered parcel compartment in his or her delivered mail compartment—a key that has been placed in the delivered mail compartment of the postal patron by a USPS delivery person who has inserted into a delivered parcel compartment one or more parcels (that will not fit in the patron&#39;s delivered mail compartment)—a key that can be used to open (on a onetime basis) the parcel compartment where the patron&#39;s parcel or parcels are waiting to be retrieved. 
     Within the interior of a CBU of the type having left and right master loading doors, are left and right shelved cabinet assemblies that define the left and right stacks of mail and parcel compartments, respectively. The left stack of compartments is separated from the right stack of compartments by a vertically extending central structure which prevents items from the left compartments from being mixed with items from the right compartments, and vice versa. Latch and lock components for retaining the left and right master loading doors closed and locked often are connected to the central structure which couples the left and right shelved cabinet assemblies that define the left and right stacks of delivered mail and delivered parcel compartments. 
     The interior of a cluster box unit also usually has a compartment for temporarily holding “outgoing mail,” namely mail that is to be collected and processed by USPS personnel for delivery to other addresses and locations. Postal patrons who have outgoing mail to be collected and processed by the USPS may insert their outgoing mail into the outgoing mail compartment through a slot (that usually is protected by a louver) formed through one of the master loading doors, often the right master loading door. 
     A variety of locking systems and lock components have been proposed for retaining the master loading doors of a CBU closed and locked against unauthorized access (i.e., the master loading doors are only to be opened by authorized USPS personnel), and for retaining in closed and locked positions the individual door-within-a-door closures that provide individual access to the various delivered mail, delivered parcel and outgoing mail compartments of the CBU. Some CBU proposals employ outgoing mail compartment doors that are especially sturdy and resistant to attack, and that are held closed by a lock mechanism installed thereon by USPS personnel that can be unlocked by one of a group of restricted access USPS keys. These “hardened” outgoing mail compartment doors (sometimes referred to as “delivery doors” or as “collection doors”), when unlocked and opened, not only permit outgoing mail to be removed from the associated outgoing mail compartment but also permit the latches of the master loading doors to be released so the master loading doors can be opened to enable USPS personnel to insert mail and parcels into the delivered mail and delivered parcel compartments. 
     When a parcel is inserted into one of the delivered parcel compartments, the USPS delivery person locks the associated parcel door and inserts the key of the parcel door into the delivered mail compartment of the postal patron to whom the parcel is addressed. When the postal patron finds a parcel door key in his or her delivered mail compartment, he or she uses the key to open the associated parcel compartment and retrieves the parcel. When a parcel door key is turned to unlock the door of a parcel compartment, the lock mechanism of the parcel door “traps” or retains the key and prevents the tenant from relocking the parcel door, which can only be locked by USPS personnel. Key retaining lock mechanisms designed for use with delivered parcel lockers in the manner just described are known. 
     Although a number of cluster box unit proposals have been approved by the USPS and have functioned satisfactorily where installed to serve the mail and parcel delivery needs of a wide variety of multi-unit dwelling facilities, a need nonetheless remains for cluster box units designed to provide improved and enhanced security, corrosion resistance and longevity of service—cluster box units that are stronger and more resistant to unauthorized entry while, at the same time, being formed from reasonably priced, relatively easy to assemble components that can be serviced, maintained and replaced with a minimum of down time when damaged due to normal wear and tear, accident, vandalism or attack are needed. In these and other factors that merit consideration during product design, prior art proposals have left room for improvement. 
     The present invention addresses these and other needs and objectives as will become apparent from the disclosure that follows. 
     It is accordingly a primary aspect of the invention to provide a cluster box units which incorporates a wide variety of features that enhance security and attack resistance while utilizing components that can be assembled quickly and easily, and that can be serviced as needed in order to keep the unit functioning properly throughout its service life that last many years. Included among the many features disclosed herein are protective enclosure improvements, internal cabinetry improvements, door, hinge and locking system improvements, and a host of other features that are not found in present day cluster box units—features that are intended to provide units that function smoothly throughout lengthy service lives, that resist corrosion, offer good appearances, and that shield mail and parcels from inclement weather and from unauthorized access or attack. 
     The cluster box unit provides outgoing mail compartment doors that are heavily constructed and rigidly reinforced, with the locks carried thereon protectively shielded, and with the extensible bolts of these locks being engaged by brackets that not only lock the outgoing mail compartment doors but also the master loading doors of the cluster box units. 
     Master loading doors are provided with hinges that extend the full height of the doors—hinges that are defined by pivotally interfitting elements of extrusions that very sturdily mount the master loading doors, that prevent prying or bending the doors in the vicinities of their hinges. The interfitting elements of the hinge extrusions also permit the doors to be installed on their surrounding door frames quickly and easily, and permit one or both of the master loading doors to be quickly and easily removed for service or replacement when necessary. 
     SUMMARY OF THE PRESENTLY CLAIMED INVENTION 
     The foregoing needs have been satisfied to a great extent by the present invention wherein, in one aspect an apparatus and method is provided that incorporates a protective enclosure, internal cabinetry, door, hinge and locking system, and a host of other features that are not found in present day cluster box units—features that are intended to provide units that function smoothly throughout lengthy service lives, that resist corrosion, offer good appearances, and that shield mail and parcels from inclement weather and from unauthorized access or attack. 
     Another aspect of the invention provides outgoing mail compartment doors that are heavily constructed and rigidly reinforced, with the locks carried thereon protectively shielded, and with the extensible bolts of these locks being engaged by brackets that not only lock the outgoing mail compartment doors but also the master loading doors of the cluster box units. 
     Yet another aspect of the invention provides master loading doors having hinges that extend the full height of the doors—hinges that are defined by pivotally interfitting elements of extrusions that very sturdily mount the master loading doors, that prevent prying or bending the doors in the vicinities of their hinges. The interfitting elements of the hinge extrusions also permit the doors to be installed on their surrounding door frames quickly and easily, and permit one or both of the master loading doors to be quickly and easily removed for service or replacement when necessary. 
     In accordance with the invention, one aspect of the invention provides a latch system for a cluster box mail delivery unit, the cluster box mail delivery unit including a top wall, a bottom wall, two sidewalls, a back side, and a front face having at least one door, comprising an upstanding extrusion having a front wall; a plurality of front wall openings formed through the front wall; a slide member movably mounted adjacent the upstanding extrusion; a plurality of slide openings formed through the slide member and disposed proximal the front wall openings; a plurality of wear plates mounted to the front wall and disposed proximal the front wall openings; an operating handle connected to the slide member to align the slide openings with the front wall openings; and a plurality of latch formations configured to align and pass through the front wall openings and slide openings, the upstanding extrusion being mounted to an internal surface of the cluster box mail delivery unit spanning the top wall and the bottom wall and the latch formations being mounted to the at least one door of the cluster box mail delivery unit. 
     Another aspect of the invention provides a cluster box mail delivery unit locking system for retaining a closure in a closed position, comprising a cam latch connectable to the closure for pivotal movement about a first pivot axis between a latched position and an unlatched position, the system including a strike for engaging with the cam latch in the latched position, and disengaging from the cam latch in the unlatched position; a key-tumable member connectable to the closure and connected to an operating arm for pivoting the operating arm relative to the closure about a second pivot axis spaced from the first pivot axis between a normal position and an operated position; a linkage connectable to the operating arm and the cam latch for pivoting the cam latch from the latched position to the unlatched position in response to pivotal movement of the operating arm from the normal position to the operated position, and for pivoting the cam latch from the unlatched position to the latched position in response to pivotal movement of the operating arm from the operated position to the normal position; a mounting member connectable to the closure for defining an externally threaded surface that extends substantially concentrically about the first pivot axis; and, an internally threaded passage formed through the cam latch and configured to be threaded onto the externally threaded surface of the mounting member for mounting the cam latch on the closure for pivotal movement between the latched and unlatched positions. 
     Yet another aspect of the invention provides a method of accessing a cluster box mail delivery unit including a top wall, a bottom wall, two sidewalls, a back side and a front face having at least one door with a pivotable end and lockable end, comprising reaching through an opening of the at least one door; grasping an operating handle connected to a slide having a plurality of slide openings, wherein, the slide substantially spans the top wall to the bottom wall of the cluster box mail delivery unit, and the at least one door has a plurality of latch formations disposed proximal the lockable end; moving the operating handle to simultaneously move the slide, and thereby position the slide openings to allow the latch formations to pass therethrough; and pivoting the at least one door to an open position. 
     Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing front, top and right side portions of one form of cluster box unit (CBU) mail delivery receptacle. 
         FIG. 2  is a front elevational view of  FIG. 1  without the support pedestal. 
         FIG. 3  is a perspective view showing front, top and right side portions of a second embodiment of cluster box unit mail delivery receptacle. 
         FIG. 4  is a front elevational view of  FIG. 3  without the support pedestal. 
         FIG. 5  is a perspective view showing front, top and right side portions of a third embodiment of cluster box unit mail delivery receptacle. 
         FIG. 6  is a front elevational view of  FIG. 5  without the support pedestal. 
         FIG. 7  is a perspective view showing front, top and right side portions of a fourth embodiment of cluster box unit mail delivery receptacle. 
         FIG. 8  is a front elevational view of  FIG. 7  without the support pedestal. 
         FIG. 9  is a perspective view of the CBU of  FIGS. 1 and 2  with left and right master loading doors thereof in open positions, with door-within-a-door elements of the master loading doors locked closed so as to pivot with other elements of the master loading doors when the master loading doors move between their closed and open positions. 
         FIG. 10  is a perspective view on an enlarged scale showing selected front and left portions of the left master loading door of the CBU of  FIGS. 1 and 2 . 
         FIG. 11  is a perspective view on an enlarged scale showing selected front and left portions of the right master loading door of the CBU of  FIGS. 1 and 2 . 
         FIG. 12  is a perspective view showing selected front and right portions of the right master loading door of the CBU of  FIGS. 1 and 2  including a hooded louvered mail slot through which outgoing mail is inserted into an outgoing mail compartment located behind the right master loading door. 
         FIG. 13  is a cross-sectional view on an enlarged scale taken along line  13 - 13  of  FIG. 2  with the left master loading door in its closed, locked position. 
         FIG. 14  is a cross-sectional view on an enlarged scale taken along line  14 - 14  of  FIG. 9  with the left master loading door in its fully open position. 
         FIG. 15  is a perspective view of a rectangular frame structure that surrounds the master loading doors of the CBU of  FIGS. 1-2 . 
         FIGS. 16-19  are cross-sectional views taken along lines  16 - 16 ,  17 - 17 ,  18 - 18  and  19 - 19  of  FIG. 15 . 
         FIG. 20  is an exploded view on an enlarged scale showing components of a corner joint of the rectangular frame structure of  FIG. 15 . 
         FIG. 21  is a perspective view showing in assembled form the components of  FIG. 20 . 
         FIG. 22  is a perspective view of portions of the master loading doors of the CBU of  FIGS. 1-2  showing how the depicted door portions come closely together when the left and right master loading doors are into engaging relationship when closed. 
         FIG. 23  is a cross-sectional view on an enlarged scale taken along line  23 - 23  of  FIG. 22 . 
         FIG. 24  is a schematic top view showing portions of the left and right master loading doors pivoted to an open position. 
         FIG. 25  is an exploded perspective view showing door-within-a-door elements of the right master loading door of the CBU of  FIGS. 1-2  separated from a frame assembly of the right master loading door, and showing a hinge rod for pivotally coupling the door-within-a-door elements to the frame assembly. 
         FIG. 26  is a perspective view on an enlarged scale of an upper right corner region of the frame assembly of  FIG. 25 , with the view permitting a recess to be seen that carries a hex nut threaded onto an upper end region of the hinge rod to secure the hinge rod in assembled position. 
         FIG. 27  is an exploded perspective view showing selected portions of the frame assembly of the right master loading door together with portions of a first form of compartment door that may be used with the frame assembly, with the compartment door in closed position in a forwardly facing recess defined by elements of the frame assembly. 
         FIG. 28  is a cross-sectional view taken along line  28 - 28  of  FIG. 27 . 
         FIG. 29  is a cross-sectional view taken along line  29 - 29  of  FIG. 27 . 
         FIG. 30  is an exploded perspective view showing selected portions of the frame assembly of the left master loading door together with portions of the first form of compartment door in closed position in a forwardly facing recess defined by elements of the frame assembly. 
         FIG. 31  is a cross-sectional view taken along line  31 - 31  of  FIG. 30 . 
         FIG. 32  is a cross-sectional view taken along line  32 - 32  of  FIG. 30 . 
         FIG. 33  is an exploded perspective view showing selected portions of the frame assembly of the right master loading door together with portions of a second form of compartment door that may be used with the frame assembly, with the compartment door in closed position in a forwardly facing recess defined by elements of the frame assembly. 
         FIG. 34  is a cross-sectional view taken along line  34 - 34  of  FIG. 33 . 
         FIG. 35  is a cross-sectional view taken along line  35 - 35  of  FIG. 33 . 
         FIG. 36  is an exploded perspective view showing selected portions of the frame assembly of the left master loading door together with portions of the second form of compartment door in closed position in a forwardly facing recess defined by elements of the frame assembly. 
         FIG. 37  is a cross-sectional view taken along line  37 - 37  of  FIG. 36 . 
         FIG. 38  is a cross-sectional view taken along line  38 - 38  of  FIG. 36 . 
         FIG. 39  is a perspective view showing selected portions of a master loading door frame assembly including four types of horizontally extending bars that may be employed as elements of the frame assembly. 
         FIG. 40  is a cross-sectional view taken along line  40 - 40  of  FIG. 39 . 
         FIG. 41  is a cross-sectional view taken along line  41 - 41  of  FIG. 39 . 
         FIG. 42  is a cross-sectional view taken along line  42 - 42  of  FIG. 39 . 
         FIG. 43  is a cross-sectional view taken along line  43 - 43  of  FIG. 39 . 
         FIG. 44  is a perspective view showing portions of one of the horizontal bars of  FIGS. 40 ,  41  and  43  carrying a weather strip closed into engagement with a shelf front of one the compartments of the CBU of  FIGS. 1-2 . 
         FIG. 45  is a cross-sectional view taken along line  45 - 45  of  FIG. 44 . 
         FIG. 46  is a perspective view showing portions of the horizontal bar of  FIG. 42  carrying a weather strip closed into engagement with a strengthened shelf front of one the compartments of the CBU of  FIGS. 1-2 . 
         FIG. 47  is a cross-sectional view taken along line  47 - 47  of  FIG. 46 . 
         FIG. 48  is a perspective view showing portions of one of the delivered mail compartment doors that forms a door-within-a-door element of one of the left and right master loading doors of the CBU of  FIGS. 1-2 . 
         FIG. 49  is a top view of  FIG. 48 . 
         FIG. 50  is a left end view of  FIG. 48 . 
         FIG. 51  is a right end view of  FIG. 48 . 
         FIG. 52  is a cross-sectional view taken along line  52 - 52  of  FIG. 48 . 
         FIG. 53  is a cross-sectional view taken along line  53 - 53  of  FIG. 48 . 
         FIG. 54  is a perspective view on an enlarged scale of a cam lock cam and portions of a strike that can be engaged by the cam to lock in closed position one of the delivered mail compartment doors of the CBU of  FIGS. 1-2 . 
         FIG. 55  is a cross-sectional view taken along line  55 - 55  of  FIG. 54 . 
         FIG. 56  is a side view of the cam and a cross-sectional view of the strike taken along line  56 - 56  of  FIG. 55 , but with latching portions of the cam extending through a elongate slot-like receiving opening of the strike, as the cam appears when pivoted to its latched position by a cam lock of a delivered mail compartment door. 
         FIG. 57  is a view thereof similar to  FIG. 56  but with the cam turned a quarter-turn to disengage the strike, as the cam appears when pivoted to its unlatched position by a cam lock of a delivered mail compartment door. 
         FIG. 58  is a perspective view showing front and left portions of the smaller of two delivered parcel compartment doors that forms a door-within-a-door element of one of the master loading doors of the CBU of  FIGS. 1-2 . 
         FIG. 59  is a cross-sectional view taken along line  59 - 59  of  FIG. 58 . 
         FIG. 60  is a perspective view showing rear and right portions of the larger of two delivered parcel compartment doors that forms a door-within-a-door element of one of the master loading doors of the CBU of  FIGS. 1-2 . 
         FIG. 61  is a rear elevational view of portions of a delivered parcel compartment door showing a guard assembly that may be used to protectively enclose components of one form of locking system for retaining the parcel door in closed position. 
         FIG. 62  is a rear elevational view of portions of a delivered parcel compartment door showing a different guard assembly that may be used to protectively enclose components of another form of locking system for retaining the parcel door in closed position. 
         FIG. 63  is a perspective view of one embodiment of locking system for delivered parcel compartment doors, with a cam thereof pivoted to a latched position. 
         FIG. 64  is another perspective view thereof, but with cam thereof pivoted to an unlatched position. 
         FIG. 65  is yet another perspective view thereof, with the cam pivoted to a latched position wherein latching portions of the cam extend through an elongate slot-like receiving formation of a strike. 
         FIG. 66  is a perspective view similar to  FIG. 65  showing an alternate form of locking system for delivered parcel compartment doors, with a pair of linkage connected cams thereof pivoted to latched positions wherein latching portions of the cams extend through elongate slot-like receiving formations of a strike. 
         FIG. 67  is a perspective view showing front, top and left end portions of an outgoing mail compartment door which serves as a door-within-a-door element of the right master loading door of the CBU of  FIGS. 1-2 , with a bolt of a USPS lock mounted on the back of the door retracted. 
         FIG. 68  is a top view of  FIG. 67 . 
         FIG. 69  is a left end view of  FIG. 67 . 
         FIG. 70  is a right end view of  FIG. 67 . 
         FIG. 71  is a perspective view showing rear, top and left end portions of  FIG. 67 , with the bolt of the door-carried lock retracted, and with a strike that can be engaged by the bolt of the door-carried lock spaced from the rear of the door, as occurs when the door is only slightly opened, or when the door is being pivoted toward a closed position. 
         FIG. 72  is a perspective view similar to  FIG. 71  but with the bolt of the door-carried lock extended through a receiving opening of the strike, as occurs when the door is locked in closed position. 
         FIG. 73  is a perspective view showing front, top and left side portions of a protective outer enclosure of the CBU of  FIGS. 1-2 . 
         FIG. 74  is a right side view of  FIG. 73 . 
         FIG. 75  is a cross-sectional view taken along line  75 - 75  of  FIG. 73 . 
         FIG. 76  is a cross-sectional view on an enlarged scale taken along line  76 - 76  of  FIG. 73 . 
         FIG. 77  is a cross-sectional view on an enlarged scale taken along line  77 - 77  of  FIG. 73 . 
         FIG. 78  is a cross-sectional view on an enlarged scale taken along line  78 - 78  of  FIG. 73 . 
         FIG. 79  is a cross-sectional view taken along line  79 - 79  of  FIG. 73 . 
         FIG. 80  is a perspective view showing front and right portions of the CBU of  FIGS. 1-2  with the delivered mail compartment doors, the delivered parcel compartment doors and the top of the protective outer enclosure thereof removed to permit interior left and right shelved cabinet features of the CBU to be seen. 
         FIG. 81  is an exploded perspective view on an enlarged scale showing features of the left and right shelved cabinets of  FIG. 80 . 
         FIG. 82  is a top view of left and right shells of the shelved cabinets of  FIG. 80 , and showing front and rear connection members extending therebetween. 
         FIG. 83  is a perspective view, on an enlarged scale, of one of the rear connection members that extends between rear portions of the left and right shells. 
         FIG. 84  is a perspective view, on an enlarged scale, of portions of a front connection member that extends between front portions of the left and right shells. 
         FIG. 85  is a perspective view of portions of upper and lower portions of the left and right shells coupled by the front and rear connection members. 
         FIG. 86  is a perspective view showing in broken lines portions of the CBU of  FIGS. 1-2 , and in solid lines latching elements carried on the front connection member and on the master loading doors that cooperate to latch the master loading doors in closed position. 
         FIG. 87  is a front elevational view on an enlarged scale of latching elements of  FIG. 86  that are carried on the front connection member. 
         FIG. 88  is a perspective view showing central portions of the latching elements of  FIG. 87 . 
         FIG. 89  is a perspective view showing one of the components of the latching elements of  FIG. 88 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention provide an apparatus and method which incorporates a protective enclosure, internal cabinetry, door, hinge and locking system. Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Four types of multiple-box mail delivery receptacles are depicted in sequence in  FIGS. 1 ,  3 ,  5  and  7 . These multiple box receptacles, referred to in the art as “cluster box units” or individually as a “CBU,” are indicated generally by the numerals  100 ,  1100 ,  2100 ,  3100 . The CBU mail delivery receptacles  100 ,  1100 ,  2100 ,  3100  have generally rectangular protective outer cabinets  110 ,  1110 ,  2110 ,  3110  that are supported atop pedestal type stands  120 ,  1120 ,  2120 ,  3120 . In  FIGS. 2 ,  4 ,  6  and  8 , the cluster box units  100 ,  1100 ,  2100 ,  3100  are depicted without their stands. 
     Because the four cluster box unit mail delivery receptacles  100 ,  1100 ,  2100 ,  3100  are assembled from components that are quite similar (i.e., components that tend to differ only in size if the components are not precisely identical), the construction and operation of all four of these cluster box units will be apparent from the description that follows even though, in some instances, the description concentrates principally on features of a selected one of the cluster box units, namely the unit  100 . 
     In the drawings and in the description that follows, “corresponding reference numerals” (i.e., reference numerals that differ by magnitudes of one, two or three thousand) are used to designate components of the cluster box units  100 ,  1100 ,  2100 ,  3100  that correspond in function—many of which components differ only slightly in dimension unless they are, in fact, completely identical. Utilizing in the drawings reference numerals that correspond to designate components that correspond in function, permits corresponding components to be identified so their corresponding functions will be understood without requiring that the text include repetitive descriptions. Thus, unless explained to the contrary, if the reference numeral  101  is used to designate a component of the cluster box unit  100 , the reader should understand that usages of the corresponding numerals  1101 ,  2101 ,  3101  (where found in the drawings) designate components of the cluster box units  1100 ,  2100 ,  3100  that correspond in function to the component  101  of the cluster box unit  100 ; and, unless explained to the contrary, if the reference numerals  102  is used to designate a component of the cluster box unit  100 , the reader should understand that usages of the corresponding numerals  1102 ,  2102 ,  3102  (where found in the drawings) designate components of the cluster box units  1100 ,  2100 ,  3100  that correspond in function to the component  102  of the cluster box unit  100 ; and so on, for other so-called “corresponding” reference numerals found in the drawings that differ by magnitudes of one, two or three thousand. 
     Referring variously to  FIGS. 1-8  (an expression that will be understood by the reader to mean that some views, such as  FIGS. 1-2 , should be referred to insofar as features of the CBU  100  are concerned; that other views, such as  FIGS. 3-4 , should be referred to insofar as features of the CBU  1100  are concerned; that other views, such as  FIGS. 5-6 , should be referred to insofar as features of the CBU  2100  are concerned; and, that still other views, such as  FIGS. 7-8 , should be referred to insofar as features of the CBU  3100  are concerned), the cabinets  110 ,  1110 ,  2110 ,  31   10  have left front door assemblies  130 ,  1130 ,  2130 ,  3130  and right front door assemblies  140 ,  1140 ,  2140 ,  3140 . U.S. Postal Service (USPS) personnel commonly refer to the left front door assemblies  130 ,  1130 ,  2130 ,  3130  as “left master loading doors,” and to the right front door assemblies  140 ,  1140 ,  2140 ,  3140  as “right master loading doors.” 
     Various optional features may be incorporated in the design of the protective outer enclosures  110 ,  1110 ,  2110 ,  3110  and/or in the design of other components of the cluster box units  100 ,  1100 ,  2100 ,  3100  such as the master loading doors  130 ,  140 ,  1130 ,  1140 ,  2130 ,  2140 ,  3130 ,  3140  to alter exterior and interior appearances of the CBUs  100 ,  1100 ,  2100 ,  3100  and their components. Likewise, various optional functional, utilitarian, structural and operational features also may be incorporated into the CBUs  100 ,  1100 ,  2100 ,  3100 . 
     The left master loading doors  130 ,  1130 ,  2130 ,  3130  are provided with hinges (indicated generally by the numerals  151 ,  1151 ,  2151 ,  3151 ) along their left sides, and the right master loading doors  140 ,  1140 ,  2140 ,  3140  are provided with hinges (indicated generally by the numerals  161 ,  1161 ,  2161 ,  3161 ) along their rights sides, so that master loading doors may be pivoted away from door closed positions depicted variously in  FIGS. 1-8  to door open positions that provide unobstructed access to the interiors of the cabinets  110 ,  1110 ,  2110 ,  3110 . For example, reference is made to  FIG. 9  wherein the master loading doors  130 ,  140  of the cluster box unit  100  are shown pivoted to fully open positions that permit mail to be delivered into left and right mail compartments designated by numerals  131 ,  141 , and so that parcels too large to fit into the mail delivery compartments  131 ,  141  can be delivered into left and right parcel compartments designated by numerals  133 ,  143 . 
     Referring still to  FIG. 9 , the interior of the CBU  100  also is provided with an outgoing mail compartment  145  at a location situated between an upper region of the right side of the interior of the CBU  100  where the right mail delivery compartments  141  are found, and a lower region of the right side of the interior of the CBU  100  where the right parcel delivery compartment  143  is found. So-called “outgoing mail,” namely mail that is to be collected and processed by USPS personnel for delivery to other addresses and locations just as though it had been deposited in and collected from a conventional USPS mail collection box, can be inserted into the outgoing mail compartment  145  through a mail slot  129  (see  FIGS. 11 and 12 ) of a louvered mail slot structure  150  that is defined by components of the right master loading door  140 . 
     The left and right delivered mail compartments  131 ,  141  (depicted in  FIG. 9 ) may be accessed by opening delivered mail compartment doors  220  when the master loading doors  130 ,  140  are closed and locked. The delivered mail compartment doors  220  have door-within-a-door relationships with the master loading doors  130 ,  140  (i.e., the doors  220  are elements of the master loading doors  130 ,  140 —elements that can pivot relative to the frameworks of the master loading doors, such as the framework  180  depicted in  FIG. 26 ). A discussion of features of the preferred form of construction for the doors  220  is provided later herein in conjunction with  FIGS. 29-38  and  48 - 52 . A discussion of locking systems for the doors  220  is provided later herein in conjunction with  FIGS. 55-57 . 
     The left and right delivered parcel compartments  133 ,  143  (depicted in  FIG. 9 ) may be accessed by opening the delivered mail compartment doors  230  when the master loading doors  130 ,  140  are closed and locked. The delivered mail compartment doors  230  have door-within-a-door relationships with the master loading doors  130 ,  140  (i.e., the doors  230  are elements of the master loading doors  130 ,  140 —elements that can pivot relative to the frameworks of the master loading doors, such as the framework  180  depicted in  FIG. 26 ). A discussion of features of the preferred form of construction for the doors  230  is provided later herein in conjunction with  FIGS. 33-38  and  58 - 62 . 
     The outgoing mail compartment  145  (depicted in  FIG. 9 ) may be accessed by opening an outgoing mail compartment door  240 . The outgoing mail compartment door  240  has a door-within-a-door relationship with the right master loading door  140 , and can pivot relative to the framework  180  of the right master loading door  140  (see  FIG. 26 ). A discussion of features of the preferred form of construction for the door  240 , and of a locking system preferably utilized to lock the door  240 , is provided later herein in conjunction with  FIGS. 67-72 . 
     Again, referring to  FIG. 9 , located to the left of the outgoing mail compartment  145  and situated between an upper region of the left side of the interior of the CBU  100  where the left mail delivery compartments  131  are found and a lower region of the left side of the interior of the CBU  100  where the left parcel delivery compartment  133  is found, is a narrow spare compartment  135  that may be used by USPS personnel for a variety of purposes, for example to retain maintenance records pertaining to the CBU  100 , or for other purposes such as storing spare mail and parcel door keys that may be needed as replacements for lost keys at some future time. The narrow spare compartment  135  provides a storage area that is secure from being accessed by those who make use of the left and right mail and parcel compartments  131 ,  141 ,  133 ,  143  because, when the left and right master loading doors  130 ,  140  are locked closed, the spare compartment  135  cannot be accessed from outside the CBU  100  through any of the openings formed through the master loading doors  130 ,  140 . 
     Whereas the cluster box units  100 ,  2100  have both left and right delivered parcel compartments served by left and right parcel doors  130 ,  2130 , respectively, the cluster box unit  1100  has only a right delivered parcel compartment served by a right parcel door  1130 , and the cluster box unit  3100  has only a left delivered parcel compartment served by a left parcel door  3130 . As those who are skilled in the art will readily understand, other cluster box unit configurations (employing delivered mail compartments and delivered parcel compartments that differ in size, number and location from those depicted in the drawings hereof) are possible—configurations that embody many of the inventive features hereof. For example, as a comparison of the cluster box units  100  and  1100  will illustrate, a plurality of delivered mail compartments can be substituted for one of the delivered parcel compartments, or vice versa; and, as a comparison of the cluster box units  2100 ,  3100  will illustrate, the sizes of delivered mail compartments and of other features of the cluster box units disclosed herein can differ while still employing many of the same inventive features. 
     A horizontally extending extrusion  137  (see  FIGS. 1 ,  2  and  10 ) of the left master loading door  130  serves to cover the front of the spare compartment  135  when the left door  130  is closed. The extrusion  137  preferably is configured to provide an exterior appearance like that which is afforded by a corresponding horizontally extending extrusion  147  of the right master loading door  140 —it being seen in  FIGS. 11 and 12  that the right master loading door extrusion  147  has an elongate, slot-like opening formed therethrough, namely the opening of the slot structure  150  through which outgoing mail is inserted into the outgoing mail compartment  145 . A hood element  149  depicted in  FIGS. 1 ,  2  and  12 , shields the slot  150  to prevent rain and snow from passing therethrough. 
     Referring to  FIGS. 1-8 , it will be seen that the cluster box units  100 ,  1100 ,  2100 ,  3100  have substantially identical louvered slot structures  150 ,  1150 ,  2150 ,  3150  defined cooperatively by identical right master loading door extrusions  147 ,  1147 ,  2147 ,  3147  and identical hood elements  149 ,  1149 ,  2149 ,  3179 . It should be noted that the features of and the appearance and construction of the louvered mail slot structures  150 ,  1150 ,  2150 ,  3150 ; of the identical extrusions  137 ,  147 ,  1137 ,  1147 ,  2137 ,  2147 ,  3137 ,  3147 ; and of the identical hood elements  149 ,  1149 ,  2149 ,  3149  are not limited to those shown in  FIGS. 1-8 . 
     Referring principally to  FIGS. 2 ,  4 ,  6  and  8 , the left master loading doors  130 ,  1130 ,  2130 ,  3130  have rectangular configurations bordered by left uprights  132 ,  1132 ,  2132 ,  3132 ; right uprights  134 ,  1134 ,  2134 ,  3134 ; top bars  136 ,  1136 ,  2136 ,  3136 ; and bottom bars  138 ,  1138 ,  2138 ,  3138 . The right master loading doors  140 ,  1140 ,  2140 ,  3140  have rectangular configurations bordered by right uprights  142 ,  1142 ,  2142 ,  3142 ; left uprights  144 ,  1144 ,  2144 ,  3144 ; top bars  146 ,  1146 ,  2146 ,  3146 ; and bottom bars  148 ,  1148 ,  2148 ,  3148 . 
     The left uprights  132 ,  1132 ,  2132 ,  3132  of the left master loading doors  130 ,  1130 ,  2130 ,  3130  preferably are defined by extrusions of identical cross-section that extend substantially the full heights of the left master loading doors  130 ,  1130 ,  2130 ,  3130 . The left upright extrusions  132 ,  1132 ,  2132 ,  3132  which are of uniform cross-section along their lengths, define halves of hinges  151 ,  1151 ,  2151 ,  3151  that pivotally mount the left master loading doors  130 ,  1130 ,  2130 ,  3130  for pivotal movement (about an axis that is designated by the numeral  51  in  FIGS. 13 ,  14 ,  18  and  24 ) between their closed positions as depicted in  FIGS. 1-8  and  13 , and open positions of the left master loading door  130  depicted in  FIGS. 9 ,  14  and  24 . The right master loading doors  140 ,  1140 ,  2140 ,  3140  are mounted for pivotal movement in the same way (about an axis that is designated by the numeral  52  in  FIGS. 19 and 24 ) between their closed positions as depicted in  FIGS. 1-8 , and open positions depicted in  FIGS. 9 and 24 . 
     Referring to  FIG. 13  wherein a cross-section of the left upright extrusion  132  is shown, the other half of the hinge that pivotally mounts the left master loading door  130  is defined by an extrusion  152  which has a groove  31  of complex cross-sectional configuration (also shown in  FIG. 18 ) that receives in an interfitting relationship a curved formation  32  of the left master loading door upright extrusion  132  in a manner that permits the left master loading door  130  to pivot about the axis  51  ( FIGS. 13 ,  14  and  24 ) between the closed position depicted in  FIGS. 1-8  and  13 , and open positions depicted in  FIGS. 9 ,  14  and  24 . Other views that also show the curved formation  32  include  FIGS. 30 ,  32 ,  36  and  38 . 
     Referring to  FIGS. 2 ,  4 ,  6  and  8 , it will be seen that left door frame upright extrusions  152 ,  1152 ,  2152 ,  3152  (all having the same cross-section as is depicted in  FIGS. 13 ,  14  and  18  where the cross-section of the extrusion  152  is shown) extend the full height along the left sides of the left master loading doors  130 ,  1130 ,  2130 ,  3130  to cooperate with the extrusions  132 ,  1132 ,  2132 ,  3132  to define the hinges  151 ,  1151 ,  2151 ,  3151  that extend for the full heights of the left master loading doors  130 ,  1130 ,  2130 ,  3130 . 
     Likewise, referring to  FIGS. 2 ,  4 ,  6  and  8 , it will be understood that full-height door hinges  161 ,  1161 ,  2161 ,  3161  are defined by interfitting extrusions  142 / 162  (see  FIG. 19  where the cross-section of the extrusion  162  is shown as having a complexly curved groove  41  that is a mirror image reversal of the cross-section of the curved groove  31  shown in  FIG. 18 ),  1142 / 1162 ,  2142 / 2162  and  3142 / 3162  that have cross sections that are mirror images of the cross-sections of the interfitting extrusions  132 ,  152  depicted in  FIGS. 13 ,  14 —and which permit the right master loading doors  140 ,  1140 ,  2140 ,  3140  to pivot open just as the left master loading door  130  is depicted as pivoting open due to relative movement of the extrusions  132 ,  152  in  FIG. 11  (see, for example,  FIG. 24  wherein the right master loading door  140  is shown pivoted about an axis  52  to an open position). 
     A curved formation  42  of the right master loading door upright  142  that extends into the curved groove  41  of the frame extrusion  162  is shown in  FIGS. 26 ,  27 ,  29 ,  33  and  35  to have a cross-section that is a mirror image reversal of the cross-section of the curved formation  32  of the left master loading door upright  132 . 
     Referring to  FIG. 15 , the left and right uprights  152 ,  162  which form elements of the hinges  151 ,  161  that pivotally mount the left and right master loading doors  130 ,  140  are two of the four elements of a rectangular frame  160  that surrounds the master loading doors  130 ,  140 . Top and bottom bars  166 ,  168  of generally F-shaped cross-section complete the frame  160 . In  FIGS. 16-19  the cross-sections of the four frame elements  152 ,  162 ,  166 ,  168  are depicted. 
     In  FIG. 20  an exploded view is provided showing how one of the four corner junctures of the frame  160  is formed, namely the corner juncture of the frame elements  152 ,  166  which is secured by screws  127  that extend through holes formed in the top bar  166  and are threaded into generally circular formations  128  of the cross-section of the upright  152 . In  FIG. 21 , a completed corner juncture of the frame elements  152 ,  166  is depicted. The other three corner junctures of the frame  160  are formed in the same manner, and the cluster box units  1100 ,  2100 ,  3100  have similarly configured frames  1160 ,  2160 ,  3160  (see  FIGS. 4 ,  6 ,  8 ) formed by top and bottom bars  1166 ,  1168 ,  2166 ,  2168 ,  3166 ,  3168  that join with the uprights  1152 ,  1162 ,  2152 ,  2162 ,  3156 ,  3162 . 
     The right upright  134  of the left master loading door  130 , and the left upright  144  of the right master loading door  140  are configured to move into close association with each other when the left master loading door  130  and the right master loading door  140  are pivoted to bring them to their closed positions, as depicted in  FIGS. 1 ,  2 ,  22  and  23 . Referring to  FIGS. 22-24 , it will be seen that the left upright  144  of the right master loading door  140  has a rearwardly turned hooked portion  170  that will be engaged by a forwardly turned hooked portion  171  of the right upright  134  of the left master loading door  130  such that even if a prybar or other pointed or edge tool is inserted into a space  173  (see  FIG. 23 ) between the uprights  134 ,  144  of the master loading doors  130 ,  140  in an effort to pry the closed, locked master loading doors  130 ,  140  away from each other the hooked portions  170 ,  171  will remain sealed. While the hooked portions  170 ,  171  are effective in resisting attack, their presence does nothing to impede normal opening and closing of the master loading doors  130 ,  140 , as is depicted in  FIG. 24 . 
     Thus, if a pry bar (not shown) is inserted into the space  173  in an effort to widen the space  173 , the hooked portions  170 ,  171  will engage—which effectively strengthens the resistance of the CBU  100  to attack. Likewise, the fact that the space  173  is backed by a rightwardly extending portion  175  (see  FIG. 23 ) of the upright  134 , and the fact that the wide front face  177  (see  FIG. 23 ) of the upright  144  is rigidified by a box-like cross-section  176  also help to defeat and fend off attack if force is applied to the master loading doors  130 ,  140  in an attempt to gain entry to the interior of the CBU  100 . 
     In as much as the master loading doors  1130 ,  1140 ,  2130 ,  2140 ,  3130 ,  3140  have uprights  1134 ,  1144 ,  2134 ,  2144 ,  3134 ,  3144  that are identical to the uprights  134 ,  144  depicted in  FIGS. 22-24 , the cluster box units  1100 ,  2100 ,  3100  also are resistant to attack if efforts are made to pry open the master loading doors  1130 ,  1140 ,  2130 ,  2140 ,  3130 ,  3140  from a closed configuration. 
     Referring to  FIG. 26 , elements of one of the master loading doors, namely the right master loading door  140  of the cluster box unit  100 , are shown. The master loading door  140  has a generally rectangular framework  180  defined by the right upright  142 , the left upright  144 , the top bar  146  and the bottom bar  148 —a framework that is rigidified and strengthened by the extrusion  147  that extends centrally between the uprights  142 ,  144 . 
     Referring to  FIGS. 11 and 12 , the extrusion  147  has a somewhat complex but generally C-shaped cross-section that is uniform along the full length of the extrusion  147  (except where the mail slot  129  opens through a central upstanding web of the C-shaped cross-section of the extrusion  147 ). Upper and lower legs of the C-shaped cross-section of the extrusion are provided with downwardly opening recesses  107  that also run the full length of the extrusion  147 . Upper portions of the recesses  107  are rounded and align with holes  108  formed through the uprights  142 ,  144  (in  FIG. 12 , the holes  108  that extend through the upright  142  can be seen; and, in  FIG. 11 , the holes  108  that extend through the upright  144  can be seen). Threaded fasteners, like the screws  127  depicted in  FIG. 20 , extend through the holes  108 , are threaded into the rounded upper end regions of the recesses  107 , and are tightened in place to clamp opposite end regions of the extrusion  147  into engagement with the uprights  142 ,  144 . 
     Referring still to  FIGS. 11 and 12 , because the uprights  142 ,  144  have bar-like formations  102 ,  104  that project toward each other along the full lengths of the uprights  142 ,  144 , opposite end regions of the extrusion  147  are provided with slots  103 ,  105  (the slot  103  can best be seen in  FIG. 11 ; the slot  105  can best be seen in  FIG. 12 ) to snugly receive the bar-like formations  102 ,  104 . The interengagement of the bar-like formations  102 ,  104  with the slots  103 ,  105  maintains proper alignment of front and rear surfaces of the extrusion  147  with front and rear surfaces of the uprights  142 ,  144 , and assists the extrusion  147  in rigidifying the framework  180  ( FIG. 25 ) of the right master loading door  140 . 
     Referring to  FIG. 10 , the left master loading door  130  is similarly reinforced, rigidified and strengthened by the extrusion  137  which has the same C-shaped cross-section as the extrusion  147 , the same recesses  107  as the extrusion  147 , and the same slots (only the slot  105  is shown in  FIG. 10 ) as the slots  103 ,  105  of the extrusion  147  for receiving bar-like formations  102 ,  104  of the uprights  132 ,  134  that are identical in configuration to the bar-like formations  102 ,  104  of the uprights  142 ,  144  of the right master loading door  140 . 
     Referring briefly to  FIG. 39  where upright portions and cross-bar elements that are common to the left and right master loading doors  130 ,  140  are depicted, it will be seen that the top bars  136 ,  146  and the bottom bars  138 ,  148  of the master loading doors  130  are slotted at opposite ends in the manner that the slots  103 ,  105  are provided in opposite end regions of the extrusions  137 ,  147  (as described just above) to receive the bar-like formations  102 ,  104  of the uprights of the doors  130 ,  140 . Likewise, it will be seen that an upper zone cross-bar  185  and a lower zone cross bar  186  also are slotted at opposite ends in the manner that the slots  103 ,  105  are provided in opposite end regions of the extrusions  137 ,  147  (as described just above) to receive the bar-like formations  102 ,  104  of the uprights of the doors  130 ,  140 . 
     Referring variously to  FIGS. 1-9 ,  26  and  80 , the left master loading doors  130 ,  1130 ,  2130 ,  3130  and the right master loading doors  140 ,  1140 ,  2140 ,  3140  are provided with differing arrays of the upper and lower zone cross-bars  185 ,  186 . Referring to  FIGS. 40-43  where cross-sections of the top bars  136 ,  146 , the upper zone bars  185 , the lower zone bars  186 , and the bottom bars  138 ,  148  are shown, it will be seen that each of the bars  136 ,  138 ,  146 ,  148 ,  185 ,  186  defines one of the downwardly opening recesses  107  into which threaded fasteners (such as the threaded fasteners  127  shown in  FIG. 20 ) can be threaded (after passing through upright holes such as the holes  108  shown in  FIG. 39 ) to secure these bars to associated ones of the uprights  132 ,  142 ,  134 ,  144 . 
     Other features shared by the bar cross-sections depicted in  FIGS. 40-43  include rearwardly-facing undercut grooves  191  that are configured to receive elongate lengths of resilient weatherstrip material of any of a wide variety of commercially available types (see, for example, the weatherstrips  190  shown in  FIGS. 44-47 ); and, concave recesses  192  provided in downwardly facing surfaces  193  of forward portions of the bars  136 ,  138 ,  146 ,  148 ,  185 ,  186  that help to prevent moisture from passing rearwardly along the downwardly facing surfaces  193  from front surfaces  201  that are exposed to rain, snow, sleet and other forms of moisture inasmuch as the front surfaces  201  constitute elements of the fronts of the cluster box units  100 ,  1100 ,  2100 ,  3100 . 
     Referring to  FIGS. 10-12 , the extrusions  137 ,  147  that extend centrally across mid portions of the left and right master loading doors  130 ,  140  also have downwardly facing surfaces  193  that are provided with concave recesses  192  to deter the rearward passage of moisture along the downwardly facing surfaces  193  of the extrusions  137 ,  147 . As also will be noted in  FIGS. 10-12 , the extrusions  137 ,  147  are provided with rearwardly facing undercut grooves  191  that, like the rearwardly facing undercut grooves  191  of the bars depicted in  FIGS. 40-43 , may provide mounting locations for lengths of weatherstripping such as the weatherstrip material  190  depicted in  FIGS. 44-47 . 
     A feature unique to the lower zone bar  186 , as seen in  FIGS. 42 ,  46  and  47  is a rearwardly projecting formation  195  configured to extend into underlying relationship with front portions of one of the many shelves  113  that define the “floors” of the various delivered mail and outgoing mail compartments  131 ,  141 ,  145  (see  FIGS. 9 and 80  wherein several of the shelves  113  are visible). The projecting formation  195  of a lower zone bar  186  is moved into underlying relationship with the front region of one of the shelves  113  when the associated master loading door (that carries the lower zone bar  186 ) is pivoted to its closed position. 
     When the projecting formation  195  of one of the lower zone bars  186  underlies a front of one of the compartment-floor-defining shelves  113 , the engagement of the projecting formation  195  with portions of the front end region of the shelf  113  (or with a reinforcing bar that may be provided to stiffen the shelf front, such as the reinforcing bar  194  depicted in  FIGS. 46 and 47 ) will help to maintain alignment of the bar  186  and the associated shelf  113 , and may also help to maintain proper registry of components carried by the master loading doors with components carried by the cabinet structures of the cluster box units  100 ,  1100 ,  2100 ,  3100 . 
     Engagement of any of a variety of rearwardly extending formations of the master loading doors  130 ,  140 ,  1130 ,  1140 ,  2130 ,  2140 ,  3130 ,  3140  with shelf front portions (for example as has just been described) also can be utilized to resist attempts to gain unauthorized access to one or more of the delivered mail or outgoing mail compartments of the CBUs  100 ,  1100 ,  2100 ,  3100  by forcing portions of one or more of the shelves  113  upwardly or downwardly—for example, attempts that sometimes are made by would-be thieves who try to pry upwardly one of the shelves  113  that overlies one of the delivered parcel compartments  133 ,  143  (depicted in  FIGS. 9 and 80 ) in an effort to access one or more of the delivered mail compartments  131 ,  141 . 
     One reason why forceful attempts sometimes are made to access the contents of delivered mail compartments by working through one of the delivered parcel compartments is that the doors to the delivered parcel compartments frequently are intentionally left “unlocked,” and therefore can be opened at will—which gives would-be intruders access to lower regions of the interior of the cluster box units  100 ,  1100 ,  2100 ,  3100 . To prevent such intrusions from succeeding, the shelves  113  that overlie the delivered parcel compartments  133 ,  143  of the cluster box unit  100  (and corresponding shelves of the cluster box units  1100 ,  2100 ,  3100  that overlie the delivered parcel compartments of the cluster box units  1100 ,  2100 ,  3100 ) preferably are securely connected to the side and rear walls  701 ,  702  of the cabinet structures  700  (see  FIG. 80 ) as by rivets, by welding or other suitable fastening techniques that are not easily broken or disconnected. 
     In preferred practice, all of the shelves  113  (including such ones of the shelves  113  as may be permanently fastened to the side and rear walls  701 ,  702  of the cabinet structures  700  as by riveting, welding or other fastening techniques), and a pair of top-most shelves  13  ( FIG. 81 ) that overlie the uppermost delivered mail compartments  131 ,  141  are connected to the side and rear walls  701 ,  702  of the cabinet structures  700  by tab-like clips  710 , best seen in  FIGS. 80 and 81 . The tab-like clips  710  are formed from the material of the side and back walls  701 ,  702  of the cabinet structures  700 —material that is displaced when openings  14  ( FIG. 81 ) are stamped through the side and back walls  701 ,  702  of the cabinet structures  700 . The tab-like clips  710  project upwardly at locations spaced short distances inwardly from the side and back walls  701 ,  702 —short distances that substantially equal the thicknesses of the material that defines the downwardly turned flanges  15  of the shelves  13 ,  113 . 
     Referring to  FIG. 81 , located on opposite sides of each of the generally rectangular openings  14  are generally rectangular openings  16  that are smaller in size than the openings  14 . To assist in holding the shelves  13 ,  113  in proper positions where the downwardly-turned flanges  15  are gripped and supported by the tab-like clips  710 , convex projections  17  are formed on the flanges  15 —projections that are configured to snap into the openings  16  when the shelf flanges  15  are properly engaged by the tab-like clips  710 . The projections  17  can take any of a variety of configurations that are capable of snapping into and being retained within the openings  16  in a manner that will keep the shelves  13 ,  113  in place in the cabinet structures  700 —configurations that render it difficult to lift the shelves  13 ,  113  out of engagement with the tab-like clips  710 . 
     Referring to  FIGS. 44-47  wherein front portions of one of the shelves  13 ,  113  are shown, it will be seen that the shelves  13 ,  113  have downwardly turned front flanges  73  that are reversely bent and turned back upwardly so that each of the flanges  73  has an associated upwardly extending reach of material  74  that is joined to the downwardly turned flange  73  by a smoothly rounded bottom formation  75  that permits a postal patron to grasp his or her deliveries from compartments located beneath the shelves  13 ,  113  without being scratched by sharp edges or burrs of the downwardly turned front flanges  73  of the shelves thereabove. 
     If a selected one of the shelves  13 ,  113  is provided with a reinforcing bar such as the reinforcing bar  194  depicted in  FIGS. 46 and 47 , the reinforcing bar  194  will help to hold the shelf  13 ,  113  in place both by giving the shelf additional strength to support a heavy load of compartment contents, and by reinforcing the shelf  13 ,  113  against being pried upwardly. A forwardly projecting portion  76  of the reinforcing bar  194  extends above the upwardly turned reach of material  74  toward a position of engagement with the backside of the downwardly turned flange  73 , by which arrangement the reinforcing bar  194  is connected to the shelf front to resist upward prying of the shelf front. 
     The reinforcing bar  194  is supported by connecting its opposite end regions to side walls  701  of cabinet structures  700 , which are best seen in  FIG. 80 . To assist in connecting opposite end regions of the reinforcing bar  194  to the side walls  701 , the extrusion that forms the reinforcing bar  194  has a groove-like recess  77  that runs the full length of the reinforcing bar  194 . The recess  77  has the same configuration as the groove-like recesses  107  provided in the extrusion-formed bars  136 ,  138 ,  146 ,  148 ,  185 ,  186  depicted in FIGS.  40 - 43 —which is to say that the groove-like recess  77  has a rounded inward portion into which fasteners (preferably like the threaded fasteners  127  depicted in  FIG. 20 ) can be threaded after passing through holes (not shown) formed through the cabinet structure side walls  701 . When such fasteners are tightened in place, opposite end regions of the reinforcing bar  194  are securely connected to the associated cabinet structure  700  so the bar  194  can perform its intended function of supporting and rigidifying the front region of its associated shelf  13 ,  113 . 
     Referring to  FIGS. 80 ,  82  and  85 , the cabinet structures  700  are substantially identical, one with the other, and are held in spaced, side-by-side relationship two or more identically configured rear connector brackets  820  (see  FIG. 85 ), one of which is shown more clearly in  FIG. 83 , and by an upstanding extrusion  830  that has a substantially uniform cross-section along its length, a segment of which is depicted in  FIG. 84 . The full length of the extrusion  830  can be seen in  FIG. 9 . Segments of the extrusion  830  also are depicted in  FIGS. 87 and 88 . 
     Referring to  FIG. 83 , each of the rear connector brackets  820  has a pair of end regions that define substantially flat walls  821  that extend in one common plane, a central region that provides another substantially flat wall  823  that extends in another plane that parallels the first common plane of the flat walls  821 , and a pair of transversely extending walls  825  that couple opposite ends of the flat wall  823  to the flat walls  821 . As is best seen in  FIG. 85 , the connector brackets  820  are installed at vertically spaced locations where the flat walls  821  overlap and are rigidly connected to the rear walls  702  of the cabinet structures  700  (best seen in  FIG. 82 ), and with the transversely extending walls  825  connected to rear portions of the side walls  701  of the cabinet structures. 
     Referring to  FIG. 84 , the extrusion  730  is of generally C-shaped cross-section, and includes right and left legs  832 ,  834  connected by a front wall  835 . At locations spaced a short distance behind the front wall  835 , a pair of opposed projections  836  extend toward each other. Defined between the projections  836  and the front wall  835  is a space that is utilized to slidably house a vertically extending, vertically movable, bar-like slide, portions of which are indicated by the numeral  840  in  FIG. 87 . 
     Referring to  FIG. 87 , several openings, indicated by numerals  842 ,  843  and  844 , are formed through the front wall  835  of the extrusion  830 —openings that permit portions of the bar-like slide  840  to be seen. Metal wear plates  850  are provided along bottom ends of the openings  842 ,  843  and are held in place by rivets  845  or other suitable fastener or fastening means. An operating handle  860 , shown in  FIGS. 86-89 , has a front wall  861  that is connected by fasteners (not shown, that extends through the opening  844 ) to the slide  840  to provide a rightwardly extending finger-engageable handle formation  865  that can be grasped to manually raise the slide  840  for the purpose of causing the slide-defined openings  852 ,  853  (which openings are at least as tall as the openings  842 ,  843  that are formed through the front wall  835  of the extrusion  830 ) to more properly align with the openings  842 ,  843  so as to permit arrowhead latch formations  900  carried by the master loading doors  130 ,  140  (see  FIGS. 9 and 86 ) to pass therethrough. 
     When the enlarged heads  910  of the arrowhead latch formations  900  have passed through the extrusion-defined openings  842 ,  843  and through the slide-defined openings  852 ,  853 , the slide  840  drops back down (under the influence of the force of gravity) to a position where the slide-defined openings  852 ,  853  do not align sufficiently with the extrusion-defined openings  852 ,  853  to enable the enlarged heads  910  to move back out through the openings  852 ,  853 . By this arrangement, the left and right master loading doors  130 ,  140  are latched closed. 
     In operation, to unlatch and open the master loading doors  130 ,  140 , one must reach through an opening of the right master loading door  140  (namely the opening that normally is closed by the door-within-a-door element  240  that provides access to the outgoing mail compartment  145  depicted in  FIG. 9 ) to grasp and raise the rightwardly projecting handle  865  to raise the slide  840  to a position wherein the extrusion-carried openings  842 ,  843  and the slide-carried openings  852 ,  853  align sufficiently to permit the enlarged heads  910  of the arrowhead latching formations  900  to pass back through the openings  842 ,  843 ,  852 ,  853  as the master loading doors  130 ,  140  pivot open about the axes  51 ,  52  (see  FIG. 24 ) 
     Furthermore, to close and latch in closed position the master loading doors  130 ,  140 , the left master loading door  130  is pivoted closed slightly ahead of the right master loading door  140  so that the centrally located uprights  134 ,  144  of the master loading doors  130 ,  140  will bring their hook-shaped formations  175 ,  177  into proper interengaging relationship, as depicted in  FIG. 23 . As the doors  130 ,  140  closely approach their fully closed positions, the arrowhead latching formations  900  pass through the openings  842 ,  843  of the central extrusion  730  (see  FIG. 87 ) and into the openings  852 ,  853  of the slide  840 . As tapered upwardly and rearwardly facing surfaces of the enlarged heads  910  of the latching formations  900  engage the material of the slide  840  located at the upper ends of the slide-defined openings  852 ,  853 , continued closing movement of the doors  130 ,  140  causes the slide  840  to raise sufficiently into alignment with the extrusion-defined openings  842 ,  843  to permit the enlarged heads  910  to pass therethrough. Once the enlarged heads  910  have passed through the slide-defined openings  852 ,  853  during closing movement of the doors  130 ,  140 , the slide  840  drops down (under the influence of the force of gravity) so as to block reverse movement of the enlarged heads  910 , thereby latching the master loading doors  130 ,  140  in their closed positions. 
     Moreover, to lock the master loading doors  130 ,  140  in their closed position, the door  240  of the outgoing mail compartment  145  must be locked. The locking system for securing the outgoing mail compartment door  240  in its closed position includes a centrally located strike  950  mounted on the right side of the upstanding central extrusion  830  near the rightwardly extending portion  865  of the operating handle  860  (see  FIG. 9 ), and a high security USPS lock  500  (see  FIGS. 9 ,  67 ,  71 ,  72 ) installed on the back side of the outgoing mail compartment door  240  at a location just behind a bracket  960  that has an upper flange  966  which overlies the body  505  of the lock  500 , a lower flange  968  that underlies the body  505  of the lock  500 , and an end portion  962  spaced inwardly from the body  505  of the lock  500 . 
     Referring to  FIGS. 71 and 72  where the strike  950  is shown most clearly, it will be seen that, as the outgoing mail compartment door  240  is pivoted closed (a partially open position of the door  240  is shown in  FIG. 71 , followed by  FIG. 72  which shows a closed, locked position of the door  240 ), a forwardly-projecting end region  952  of the strike  950  is received between the body  505  of the lock  500  and the end portion  962  of the bracket  960 . When the outgoing mail compartment door  240  is closed so that the forward end region  952  of the strike  950  is positioned as just described, an appropriately configured key (not shown) can be turned in the lock  500  to extend the bolt  510  of the lock through aligned openings  954 ,  964  of the forward end region  952  of the strike  950  and the end region  962  of the bracket  960  to securely lock the door  240  of the outgoing mail compartment  145  closed. 
     When the outgoing mail compartment door  240  is locked closed in the manner just described, the right master loading door  140  also is locked closed (i.e., the door  140  cannot be opened when the door  240  is locked in closed position against the framework  180  ( FIG. 25 ) of the right master loading door); and, when the right master loading door  140  is locked closed, the left master loading door  130  also is locked closed (i.e., the left master loading door  130  cannot be opened when the upright  144  of the right master loading door  140  which overlies and blocks opening movement of the right upright  134  of the left master loading door  130 , which blocked movement is depicted in  FIG. 23  where the right door formation  170  overlies the left door formation  175 ). 
     The approach of latching closed the master loading doors of a cluster box unit by utilizing arrowhead latch formations  900  that pass through aligned openings of a slide mechanism and that are retained when the slide translates to prevent the arrowhead formations from passing back through the slide is known, as is evidenced by U.S. Pat. No. 5,794,844 issued Aug. 18, 1998, assigned to a subsidiary of the assignee of the present application. However, differences exist between the latching system of the present invention and the latching system disclosed in U.S. Pat. No. 5,794,844. 
     Referring to  FIG. 87 , one of several notable improvements and/or structural distinctions (offered by components of the latching system that releasably retains the master loading doors  130 ,  140  closed—features not found in the latching system of U.S. Pat. No. 5,794,844) is the provision of easily replaced wear plates  850  (preferably formed from steel, most preferably stainless steel) that are fastened by removable fasteners such as rivets  845  to central portions of the vertically extending central extrusion  830  (preferably formed from aluminum). The wear plates  850  underlie the extrusion-defined openings  842 ,  843  to provide upper edges that often will be engaged by bottom surfaces of the arrowhead latch formations  900  when the arrowhead latch formations move back and forth through the openings  842 ,  843  during opening and closing of the master loading doors  130 ,  140 . 
     The hard, wear resistant upper surfaces of the wear plates  850  are not worn away (by bottom surfaces of the steel arrowhead formations  900  rubbing thereacross as the master loading doors  130 ,  140  open and close) nearly as quickly as would be the much softer material of the extrusion  830  (aluminum preferably is used to form the extrusion  830 , hence aluminum is the material that defines the openings  842 ,  843 ). If the wear plates  850  deteriorate (due, for example, to repetitive engagement day-after-day as bottom surfaces of the arrowhead latch formations  900  rub across upwardly facing surfaces of the wear plates  850 ), the wear plates  850  can be quickly and easily replaced by drilling out or otherwise removing rivets  845  or other fasteners that hold the wear plates  850  in place on the extrusion  830 , and by installing new wear plates  850  on the extrusion  830  by utilizing new rivets  845  or other suitable fasteners. 
     Another of the several improvements and/or structural distinctions offered by latching system components of the cluster box units  100 ,  1100 ,  2100 ,  3100  (in comparison with latching system features disclosed in U.S. Pat. No. 5,794,844) is the smoothly configured, well positioned operating member  860  and its rightwardly extending handle formation  865  that can be located easily when one reaches through the opening of the right master loading door  140  that normally is closed by the outgoing mail compartment door  240 . The rightwardly extending handle formation  865  is located and configured so that it can easily be found when one inserts his or her hand through the door opening, but is located and configured so as to not obstruct the door opening should the USPS delivery person desire to remove mail through the door opening—mail that has accumulated in the outgoing mail compartment  145 . 
     Still another feature is the location and configuration of the rightwardly extending handle formation  865  is that, when the right master loading door  140  and the outgoing mail compartment door  240  both are closed, the handle formation  865  projects beneath the upper flange  866  (seen in  FIGS. 67-72 ) in a manner that enables the upper flange  866  of the bracket  860  to block the path of upward movement normally followed by the handle formation  865  when the handle formation is raised to elevate the slide  840  and release the arrowhead formations  900  from being retained in the openings  842 ,  843 ,  852 ,  853 . By blocking the handle formation  865  from being raised, the overlying upper flange  866  of the bracket  860  effectively prevents the master loading doors  130 ,  140  from being unlatched at times when the outgoing mail compartment door  240  is closed and locked. 
     The delivered mail compartment doors  220  are normally kept closed and locked. Cam locks  920  ( FIGS. 1 ,  2  and  48 ) that are mounted on each of the mail compartment doors  220  are operated by keys (not shown) which are maintained in the custody of postal patrons to whom the delivered mail compartments are assigned. The cam locks  920  ( FIGS. 1 ,  2  and  48 ) may take the form of conventional, commercially available cam lock assemblies intended for use in turning cams between latched and unlatched positions—which typically involves about one-quarter, one-third or one-fifth of a revolution of turning movement. While conventional, relatively small, relatively thin cams of the type normally provided with cam lock assemblies by the manufacturers of cam lock assemblies may be used to engage suitably configured strike formations to hold the mail compartment doors  220  closed, in preferred practice, heavier-duty cams (that are thicker and larger in size than the relatively thin, relatively small cams normally provided with commercially available cam lock assemblies) are employed, such as the cam  922  depicted in  FIGS. 54-57 ; or such as the cam  972  depicted in FIGS.  61  and  63 - 65 . 
     Referring to  FIGS. 54-57 , the cam  922  preferably is formed from steel (most preferably stainless steel) and has a centrally located opening  924  that receives the rear end region of a key-turnable plug  926  (FIGS.  9  and  55 - 57 ) of the cam lock assembly  920  on which the cam  922  is mounted. The cam  922  can be pivoted by the key-turnable plug  926  about an axis  925  ( FIGS. 55-57 ) of the plug  926  between an unlatched position depicted in  FIG. 57  wherein a latching formation  928  of the cam  922  is withdrawn from engagement with a suitably configured strike  930 , and a latched position depicted in  FIG. 56  wherein the latching formation  928  engages the strike  930  so as to retain the delivered mail compartment door  220  (on which the cam lock assembly  920  is mounted, as shown in  FIGS. 1 ,  2  and  48 ) in a closed, locked position (shown in  FIGS. 1-8 ). 
     While the strike  930  may take any of a wide variety of conventional configurations, and while the latching formation  928  may take any of a wide variety of configurations offered by the cams that typically are provided by the manufacturers of cam lock assemblies, in preferred practice the latching formation  928  is of hook-shaped configuration and is positioned to extend through an elongate slot-like formation  932  of the strike  930  so that, when the cam  922  is in the latched position shown in  FIG. 56 , a reversely turned projecting element  934  of the hook-shaped latching formation  928  is positioned close to and in alignment with an opening  938  of the strike  930 . By this arrangement, if the door  220  on which the cam lock assembly  920  is mounted is pried away from the strike  930  at a time when the cam  922  is in its latched position, the projecting element  934  of the latching formation  928  of the cam  922  will be drawn into the opening  938  to securely couple the cam-lock-carrying door  220  to the strike  930  to strongly resist attempts to defeat the action of the cam lock assembly  920  by prying the locked mail compartment door  220  open. 
     Cam lock assemblies  920  ( FIGS. 1 ,  2  and  48 ) selected for use on the delivered mail compartment doors  220  preferably are of the type that have keys which can be removed only when the cams they carry are pivoted to their latched positions. Selecting cam lock assemblies that have only one key removal position (i.e., their key-turned plugs must position their cams in latched positions in order for the keys to be removed from their key-turned plugs) ensures that the delivered mail compartment doors  220  cannot be left unlocked when postal patrons remove their keys from the cam locks (unless, of course, a postal patron fails to properly close the door  220  to his or her delivered mail compartment  131 ,  141  when leaving the vicinity of the cluster box unit  100 —however, this is unlikely inasmuch as the cam  922  will be seen to project from the end of the door  220 , and the door  220  will be seen to project outwardly from the front plane of the collection box unit  100  if the cam  922  is turned to the locked position at a time when the door  220  is improperly closed). 
     Cam configurations, strike configurations and other features of the type just described are disclosed in greater detail in the above-referenced Rugged Cam Lock Cases, with still other features disclosed in the non-provisional application, Ser. No. 10/879,570 entitled LINKAGE OPERATED CAM LOCK FOR A CLOSURE, filed Jun. 29, 2004, the disclosure of which is incorporated herein by reference. 
     Although the delivered mail compartment doors  220  are not normally provided with return springs to assist postal patrons in closing, and in maintaining closed, the doors  220 , return springs can be provided, if desired. One possible approach to providing return springs on compartment doors of cluster box units is illustrated in  FIGS. 59 and 60  in conjunction with the delivered parcel compartment doors  230 . 
     Turning to  FIGS. 59 and 60 , each of the delivered parcel compartment doors  230  is provided with a torsion return spring  990 . The torsion return spring  990  has an upper end region  991  (best seen in  FIG. 59 ) connected by an overlapping plate portion  993  to the parcel door  230 , and a lower end region  992  that bears against a framework element of the associated master loading door, typically one of the uprights  132 ,  134 ,  142 ,  144 . The torsion springs  990  bias the parcel doors  230  toward their closed positions—a biasing action that is needed inasmuch as the parcel compartment doors  230  normally are not locked, normally are openable at will, and might stand open (if not biased toward their closed positions) so as to admit moisture and unwanted debris into the associated parcel compartments. 
     Each of the delivered parcel compartment doors  230 ,  1230 ,  2230 ,  3230  is provided with a dual-lock, dual-key-operated locking system that permits the parcel door to be locked only by a USPS employee; and USPS employees lock a selected parcel compartment only when they insert into the selected parcel compartment a parcel that is too large to be inserted into the delivered mail compartment of the postal patron to whom the parcel is addressed. 
     Referring to  FIGS. 58 ,  63  and  64 , the dual-lock locking system installed on each parcel compartment door  230  includes a USPS installed high security lock  500  that can only be operated by a restricted access key kept continuously in the custody of USPS personnel, and a lower security cam lock assembly  570  (a commercially available cam lock) of the type that has a key-turnable plug  571 ,  4571  (see  FIGS. 63-66 ) that will permit the key of the cam lock  570  to be removed from the key-turnable plug  571 ,  4571  only when the plug  571 ,  4571  is rotated to one particular angular orientation—namely a “door locked” orientation wherein a cam  572  or cams  4572  that is/are operated by the cam lock  570  is/are pivoted into engagement with a strike formation  930  (see  FIGS. 65 ,  66 ) so as to lock the associated parcel compartment door  230  closed. 
     Thus, when a parcel compartment door  230  is closed and locked, the key of the cam lock  570  is removed from the cam lock  570  by the USPS employee who has inserted a parcel into the associated parcel compartment. The USPS employee then inserts the parcel compartment door key into the delivered mail compartment of the postal patron to whom the parcel is addressed, and the postal patron (upon finding the key) utilizes it to retrieve his or her parcel by unlocking and opening the associated parcel compartment door  230 . 
     In operation, to lock one of the parcel compartment doors  230 , a USPS employee inserts and turns a first key (namely a restricted access key that is maintained continuously in the custody of USPS personnel—usually the same key that is used by USPS personnel to operate the USPS installed high security lock  500  mounted on the outgoing mail compartment door  240 ) to operate the USPS high security lock  500  carried on the parcel compartment door  230 . When the high security lock  500  is operated by a USPS employee, this releases the mechanism of the cam lock  570  so the key of the cam lock  570  can be turned to pivot a cam  572  ( FIG. 61 ) from an unlatched position (where the cam  572  is retracted into a guard assembly  595  carried on the back side of the parcel door  230 , as depicted in  FIGS. 60 and 61 ; or where the cams  4572  are retracted into a guard assembly  4595 , depicted in  FIG. 62 ) to the latched position depicted in  FIGS. 61 ,  62  to lock the associated parcel compartment door  230 . 
     Once the parcel compartment door  230  is locked, the keys of both of the locks  500 ,  570  are removed. The key to the USPS high security lock  500  is retained by USPS personnel. The key to the lower security cam lock  570  is deposited by USPS personnel in the delivered mail compartment of the postal patron who needs to open the parcel compartment to collect his or her parcel. 
     Once the postal patron inserts and turns the key (that he or she finds in his or her delivered mail compartment) in the cam lock  570  of the parcel compartment door  230 , the high security USPS lock  500  prevents the lower security cam lock  570  from re-locking the parcel compartment door  230 , and the key of the lower security cam lock  570  (which has been turned by the postal patron to an unlocked position) is retained in the cam lock  570  until a USPS employee next takes action to relock the parcel compartment door  230  because he or she has delivered a new parcel into the associated parcel compartment  133 ,  143 . The dual-key, dual-locking system used on the parcel compartment doors  230  is (in the manner just described) put through one cycle of operation after another as new parcels (too big to be inserted into the delivered mail compartments  131 ,  141  of the postal patrons to whom the parcels are addressed) are, from time to time, delivered to the parcel compartments  133 ,  143  and retrieved by the postal patrons to whom the parcels are addressed. 
     One form of a dual-key, dual-locking system for parcel compartment doors is disclosed in U.S. Pat. No. 4,865,248 issued Sep. 12, 1989, the disclosure of which is incorporated herein by reference. A more preferred form of a dual-key, dual-locking system for use with the parcel compartment doors  230  is disclosed in the referenced non-provisional application, Ser. No. 10/879,570 entitled LINKAGE OPERATED CAM LOCK FOR A CLOSURE, filed Jun. 29, 2004, the disclosure of which is incorporated herein by reference. 
     Among the types of dual-key, dual-locking system features are linkage operated single-cam locking systems of the type depicted in FIGS.  61  and  63 - 65  hereof, and linkage operated plural-cam locking systems of the type depicted in  FIGS. 62 and 66  hereof. To provide a quick overview of these linkage operated cam locking systems, reference is made to  FIGS. 63-65  where a single cam  572  is shown, and to  FIG. 66  wherein plural cams  4572  are shown. The cams  572 ,  4572  are mounted on door-carried, post-like pedestals  573 ,  4573  to pivot between latched positions shown in  FIGS. 63 ,  65  and  66 , and an unlatched position shown in  FIG. 64 . Pivoting of the cams  572 ,  4572  between the depicted latched and unlatched positions is caused by pivotal movement of operating arms  574 ,  4574  carried on the key-turned plugs  571 ,  4571  of associated cam lock assemblies (for example, the cam lock  570  that is depicted in  FIGS. 58 and 63 ). 
     Referring to  FIGS. 65 and 66 , it will be seen that the cams  572 ,  4572  (like the cam  928  depicted in  FIGS. 54-57 ) have latching portions  928  that, when in the depicted latched positions of the cams  572 ,  4572 , preferably extend through elongate slot-like receiving formations  932  of the strikes  930 . When in the depicted latched positions shown in  FIGS. 65 and 66 , reversely turned projections  934  of the cams  572 ,  4572  reside close to and in alignment with openings  938  formed through the strikes  930  so that, if the parcel compartment doors  230  on which the cams  572 ,  4572  are supported are pried away from the depicted strikes  930 , the projections  934  will be drawn into the openings  938  to aid in securely holding the parcel compartment doors  230  closed. 
     The operating arms  574 ,  4574  pivot between normal positions depicted in  FIGS. 63 ,  65  and  66 , and an operated position shown in  FIG. 64  to cause links  576 ,  4576  (that couple the operating arms  574 ,  4574  to the cams  572 ,  4572 ) to pivot the cam  572  or the link interconnected cams  4572  between their latched positions shown in  FIGS. 63 ,  65  and  66 , and an unlatched position shown in  FIG. 64 . The operating arms  574 ,  4574  carry spring members  577 ,  4577  that interact with the retractable-extensible latch bolts  510  of the associated USPS installed high security locks  500  to control when the operating arms  574 ,  4574  can pivot from their operated positions (shown typically in  FIG. 64 ) to their normal, non-operated positions (shown in  FIGS. 63 ,  65  and  66 ) to pivot the cam  572  or cams  4572  from their unlatched positions (one of which is shown in  FIG. 64 ) to their latched positions (shown in  FIGS. 63 ,  65  and  66 ). Structural features that may be utilized in single and/or plural cam locking systems of this general type are shown in  FIGS. 65-66  accordingly. 
     The protective outer enclosure  110  that houses, surrounds and encloses the various interior components of the cluster box unit  100  has several features that merit mention. Referring to  FIGS. 73-75 , the enclosure  110  has opposed side walls  901 , a rear wall  902  and a top wall  903  that cooperate with a bottom assembly  905  to define a forwardly-opening compartment  906  into which most of the interior components of the cluster box unit  100  are inserted after the interior components are assembled. The side and rear walls  901 ,  902  are integrally formed, as by bending a sheet of metal (preferably aluminum or stainless steel) to provide right-angle corners that connect the rear wall  902  to the side walls  901 . 
     The top wall  903  is formed from a sheet of metal (preferably aluminum or stainless steel) that is bent to provide depending side and rear flanges  911 ,  912  (see  FIGS. 73-75  and  78 ) that overlie upper portions of the side and rear walls  901 ,  902 , respectively; and to provide a downwardly extending, reversely turned front portion  913  (see  FIG. 76 ) having an upwardly concave formation  914  that functions in the manner of the concave recesses  192  of the cross-bars  136 ,  138 ,  146 ,  148 ,  185 ,  186  (see  FIGS. 40-43 ) to prevent moisture from traveling rearwardly along downwardly facing surfaces, such as the downwardly facing surface  915  shown in  FIG. 76 . 
     The side walls  901  are provided with vertically extending members  918  (see  FIGS. 73 ,  75  and  77 ) that are configured to be engaged by edge portions  919  of the upstanding extrusions  152 ,  162  (see  FIGS. 18 and 19 ) of the rectangular frame  160  (see  FIG. 15 ) when the frame  160  (with the master loading doors  130 ,  140  installed thereon) is secured by threaded fasteners (not shown) to the enclosure  110  after other internal components of the cluster box unit  100  have been inserted into the interior of the enclosure  110 . All fasteners used to fasten the interior components of the cluster box unit  100  in place within the enclosure  110  are accessible only when the master loading doors  130 ,  140  are unlocked and open—an arrangement that prevents unauthorized access by those who would utilize tools to remove fasteners that hold together components of the cluster box unit  100  if such fasteners were accessible from the exterior of the unit  100 . 
     Referring to  FIG. 79 , the bottom assembly  905  includes a bottom wall member  906  having downwardly turned side and rear flanges  907 ,  908  that are fastened to the side and rear walls  901 ,  902 , and a downwardly turned front flange  909  (see  FIG. 75 ) that defines a forwardly facing surface at the bottom of the enclosure  110 . Referring to  FIGS. 73 and 79 , a pair of floor reinforcing members  919  extend forwardly-rearwardly between the downwardly turned rear and front flanges  908 ,  909  to define mounting holes  916  ( FIG. 79 ) that are accessible through bottom wall openings  917  ( FIG. 73 ) when bolts (not shown) are to be installed that connect the enclosure  110  to the pedestal support  120  ( FIG. 1 ). 
     Referring to  FIGS. 40-43 , the bars  136 ,  138 ,  146 ,  148 ,  185 ,  186  that are utilized in the frameworks of all of the master loading doors  130 ,  1130 ,  2130 ,  3130 ,  140 ,  1140 ,  2140 ,  3140  all have forwardly-facing surfaces  200 . Referring to  FIG. 25  (wherein the framework  180  of one of the master loading doors  130 ,  140 ,  1130 ,  1140 ,  2130 ,  2140 ,  3130 ,  3140  is depicted), the forwardly-facing surfaces  200  align with forwardly facing surfaces  202 ,  204  of the bar-like formations  102 ,  104  of the left and right uprights of the associated master loading door. The alignment of the surfaces  200 ,  202 ,  204  (i.e., the fact that the surfaces  200 ,  202 ,  204 ) extend in a common plane) provides a perimetrically extending backstop against which the doors  220 ,  230 ,  240  that individually serve the various delivered mail, delivered parcel and outgoing mail compartments (such as the compartments  131 ,  133 ,  141 ,  143 ,  145  depicted in  FIG. 9 ) of the cluster box unit mail delivery receptacles  100 ,  1100 ,  2100 ,  3100 . 
     Referring still to  FIG. 25 , to pivotally connect the doors  220 ,  230 ,  240  to the framework  180 , an elongate hinge rod  250  is provided that extends through aligned holes  251  formed through right end regions of the various horizontally extending bars  146 ,  148 ,  185 ,  186 , and through a right end region of the extrusion  147 . The hinge rod  250  has threaded end regions  252  onto which lock nuts  253  may be threaded. The hinge rod  250  is held in place (after being inserted through the aligned holes  251  of the framework  180  and through hinge-rod-receiving passages  255  provided in right end regions of the doors  220 ,  230 ,  240 ) by installing the lock nuts  253  on the threaded end regions  252  and tightening the nuts  253  to draw the nuts into cylindrical recesses defined by the top and bottom bars  146 ,  148  of the framework  180 , such as the cylindrical recess  256  provided in the top bar  146  as depicted in  FIG. 26 . 
     In practice, the doors  220 ,  230 ,  240  are formed as extrusions that have uniform cross-sections that extend from top to bottom of each of the doors  220 ,  230 ,  240 . However, the fact that each of the doors  220 ,  230 ,  240  preferably is formed as an extrusion that, when extruded, features a uniform cross-section from top to bottom should not be misinterpreted to mean that the doors  220 ,  230 ,  240  all have the same cross-sections; or that the extrusion-formed uniform cross-sections of the doors  220 ,  230 ,  240  remain unmodified after the extrusions that forms the various doors  220 ,  230 ,  240  are cut into appropriate lengths that correspond to the top-to-bottom dimensions of the doors  220 ,  230 ,  240 . 
     Stated in another way, blanks that are used to form the doors  220 ,  230 ,  240  may be cut from extrusions that all have the same cross-section, or from extrusions that feature a variety of different cross-sections. After door blanks are extrusion-formed and cut to appropriate lengths, the blanks may be stamped or machined or otherwise reconfigured so that, when in final form and ready for installation, they do not necessarily have cross-sections that are uniform along the top-to-bottom lengths of the doors  220 ,  230 ,  240 ,  1220 ,  1230 ,  1240 ,  2220 ,  2230 ,  2240 ,  3220 ,  3230 ,  3240  of the CBUs  100 ,  1100 ,  2100 ,  3100 . 
     Referring to  FIGS. 48-52  wherein one preferred form of delivered mail compartment door is indicated generally by the numeral  220 , it will be seen that the door  220  has an enlarged, rounded right end region  221 , an enlarged left end region  222 , and a central reach  223  that is substantially flat and of substantially uniform thickness except 1) where a generally rectangular indentation  224  has been formed at a central location to provide a forwardly facing recess  225  that can receive a compartment identification label (not shown), and 2) at a location near the left end region  222  where a rearwardly-opening groove  226  has been provided to diminish the thickness of the door in the vicinity where the cam lock  920  is mounted in an opening (not shown) formed through the door  220 . In  FIGS. 48-52 , only front portions of the cam lock  920  are depicted inasmuch as the nature of the conventional, commercially available cam lock  920  that ordinarily is mounted on the mail compartment doors  920  has been disclosed earlier herein. 
     Referring to  FIGS. 33-38  and  58 - 60 , an alternate form of door cross-section (that is thicker than the delivered mail compartment doors  220 ) is used to form the delivered parcel compartment doors  230 . The doors  230  have a cross-section that is of uniform thickness except 1) where a generally rectangular indentation  234  ( FIG. 58 ) has been formed at a central location to provide a forwardly facing recess  235  that can receive a compartment identification label (not shown), and 2) at a location near the right end region where a region  232  of diminished cross-section extends rightwardly, terminating in a relatively thin hook-like formation  233  that extends the full height of the right end region of the door  230 . The hook-shaped formation  233  ( FIG. 59 ) preferably is configured so that, from the front side of the doors  230 , the doors  230  have an appearance that is almost completely identical to the appearances of the right end regions  221  of the delivered mail compartment doors  220 . 
     In the back side of the door  230  (in the region  232  of diminished cross-section), a vertically extending sheet of metal  235  (best seen in  FIGS. 35 ,  38  and  60 ) is mounted that extends nearly, but not completely, the full height of the right end region of the door  230 . The sheet of metal  235  has a curved formation  236  ( FIG. 60 ) that encircles the hinge rod  250  ( FIG. 25 ) that pivotally mounts the doors  220 ,  230 ,  240  on the frameworks of the left and right master loading doors  130 ,  140  (and on the other master loading doors  1130 ,  1140 ,  2130 ,  2140 ,  3130 ,  3140 ). As is best seen in  FIG. 60 , a portion  237  of the curved formation  236  may be cut away to provide room for the torsion return spring  990  that biases the parcel doors  230  closed, and a portion  993  of the metal sheet  235  may be configured to receive a leg  991  of the return spring  990 . 
     Referring to  FIGS. 67-72 , the same thick cross-section used to form the parcel doors  230  also is preferably used to form the outgoing mail compartment door  240 —and a curved metal sheet  235  of the type employed on the doors  230  is used to define a hinge rod passage  255  for mounting the door  240  on the hinge rod  250 . 
     As will be apparent from the foregoing description taken together with the accompanying drawings, the cluster box units  100 ,  1100 ,  2100 ,  3100  preferably incorporate a wide variety of improvement features that enhance security and attack resistance while utilizing components that can be assembled quickly and easily, and that can be serviced as needed in order to keep the units  100 ,  1100 ,  2100 ,  3100  functioning properly for service lives that last many years. Included among the many features disclosed herein are protective enclosure improvements, internal cabinetry improvements, door, hinge and locking system improvements, and a host of other features that are not found in present day cluster box units—features that are intended to provide units that function smoothly throughout lengthy service lives, that resist corrosion, offer good appearances, and that shield mail and parcels from inclement weather and from unauthorized access or attack. 
     The cluster box units  100 ,  1100 ,  2100 ,  3100  have outgoing mail compartment doors that are heavily constructed and rigidly reinforced, with the locks carried thereon protectively shielded, and with the extensible bolts of these locks being engaged by brackets that not only lock the outgoing mail compartment doors but also the master loading doors of the cluster box units. 
     Master loading doors are provided with hinges that extend the full height of the doors—hinges that are defined by pivotally interfitting elements of extrusions that very sturdily mount the master loading doors, that prevent prying or bending the doors in the vicinities of their hinges. The intermitting elements of the hinge extrusions also permit the doors to be installed on their surrounding door frames quickly and easily, and permit one or both of the master loading doors to be quickly and easily removed for service or replacement when necessary. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example, and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. It is intended to protect whatever features of patentable novelty exist in the invention disclosed. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.