The present invention relates to shelf boxes for use in shelving racks, storage cabinets and the like, typically for the storage of small parts.  Such shelf boxes are well known in the art and are normally equipped with a stop member at the rear end of the box which is engageable with a turned down flange on the front of the shelf above the box to prevent the box from accidentally being pulled off the shelf. 
Such shelf boxes are extensively used, for example, in warehouses or parts distribution centers for the storage of large number of different types and sizes of parts.  In the typical warehouse arrangement, the storage racks or cabinets are arranged in long parallel rows with aisles therebetween.  Normally, the shelf box can be withdrawn from only one side of the rack or cabinet.  This can cause congestion when several people want access to different boxes which are disposed closely adjacent to one another in a rack or cabinet. 
Therefore, it has been found to be desirable to arrange shelf racks or storage cabinets so that the shelf boxes thereon can be removed from either side of the rack.  Thus, if a person cannot reach a box from one side of the rack, he can move around to the aisle on the opposite side of the rack and gain access to the box from that position.  This necessitates shelf boxes which can be withdrawn or opened from either side of the shelf rack or storage cabinet.  This has been impossible with prior art shelf boxes since the stop member at the rear of the cabinet which limits the extended withdrawal at one side of the rack will completely prevent withdrawal at the other side of the rack. 
Double-acting drawers, panels or other furniture members are known in the art which can be withdrawn or opened from either side of a furniture cabinet.  But such prior art devices have required the use of complicated movable stop mechanisms mounted on the cabinet for each drawer or other withdrawable member.  Such stop mechanisms are shown, for example, in the U.S. Pat.  No.  2,599,865, issued to E. G. Rudman on June 10, 1952.  These cabinet-mounted movable stop mechanisms would be completely unsuitable for a shelf box type of application, since there may be hundreds of shelf boxes in a single cabinet or rack and the provision of cabinet-mounted stop assemblies for each shelf box would result in a prohibitively expensive and complicated rack or cabinet structure, as well as a considerable waste of space. 
Furthermore, it is common in warehouse applications of shelf boxes that the rack construction permits the mounting of shelves with different spacings therebetween for accommodating shelf boxes of varying heights and, furthermore, on any given shelf, shelf boxes of varying widths may be stored.  This flexibility would be effectively destroyed if it were necessary to provide complicated rack-mounted stop assemblies for each shelf box. 
Other double-opening drawers are shown, for example, in the U.S. Pat.  No.  2,914,370, issued to E. J. Hensch et al.  on Nov. 24, 1959.  The Hensch et al.  apparatus includes latch mechanism which must be manually operated or disengaged in order to open the drawer in either direction.  Such latch mechanism adds considerable expense and complication to the construction of the mechanism, while also complicating the operation thereof.  These disadvantages would effectively prohibit operation of the Hensch et al.  type of device in a shelf box environment.  Also, the Hensch et al.  device necessitates the mounting of movable latch means on the cabinet assembly for each drawer, with the attendant disadvantages which were discussed above in connection with the Rudman patent.