Abstract: A cylindrical "shell and plug" assembly has the plug rotatably received within the shell. The plug has a central rear shaft and an engagement portion thereon. The engagement portion has a pie-shaped stop member and a pie-shaped engagement member. A connecting member rotatably held within the shell has an inner engagement that can engage the pie-shaped engagement member and an operation key slot capable of receiving an end of an operation key. The plug can be rotated until the engagement member is positioned behind the flange to retain the plug with the connecting member, and the plus can be rotated so that the engagement member can be positioned away from the flange, thereby permitting the plug to become disengaged from the connecting member.