A loading unit including an interlock assembly for attaching the loading unit to a surgical stapling instrument, the loading unit comprising: a shell member including a proximal end and a distal end, the shell member including a cylindrical portion formed on the proximal end of the shell member, the distal end of the shell member configured for supporting a staple cartridge defining a plurality of staple retaining slots arranged in a pair of concentric rows, the cylindrical portion defining an opening formed therein; and a collar member pivotally secured to the cylindrical portion, the collar member including a locking shelf selectively receivable within the opening of the cylindrical portion of the shell member, wherein: the locking shelf is spaced from the opening of the cylindrical portion when the collar member is in an open position; and the locking shelf is received through the opening in the cylindrical portion when the collar member is in a closed position, wherein the collar member includes a first end pivotally secured to the cylindrical portion of the shell member and a second end releasably securable to the cylindrical portion, and wherein the collar member is pivotable about the first end of the collar member from the closed position to the open position, wherein the first end of the collar member is pivotally secured to the cylindrical portion of the shell member by a pivot pin. a shell member including a proximal end and a distal end, the shell member including a cylindrical portion formed on the proximal end of the shell member, the distal end of the shell member configured for supporting a staple cartridge defining a plurality of staple retaining slots arranged in a pair of concentric rows, the cylindrical portion defining an opening formed therein; and a collar member pivotally secured to the cylindrical portion, the collar member including a locking shelf selectively receivable within the opening of the cylindrical portion of the shell member, wherein: the locking shelf is spaced from the opening of the cylindrical portion when the collar member is in an open position; and the locking shelf is received through the opening in the cylindrical portion when the collar member is in a closed position, wherein the collar member includes a first end pivotally secured to the cylindrical portion of the shell member and a second end releasably securable to the cylindrical portion, and wherein the collar member is pivotable about the first end of the collar member from the closed position to the open position, wherein the first end of the collar member is pivotally secured to the cylindrical portion of the shell member by a pivot pin. the locking shelf is spaced from the opening of the cylindrical portion when the collar member is in an open position; and the locking shelf is received through the opening in the cylindrical portion when the collar member is in a closed position, wherein the collar member includes a first end pivotally secured to the cylindrical portion of the shell member and a second end releasably securable to the cylindrical portion, and wherein the collar member is pivotable about the first end of the collar member from the closed position to the open position, wherein the first end of the collar member is pivotally secured to the cylindrical portion of the shell member by a pivot pin.