Abstract: Substitutable plug assemblies are removably retained at an operating position within a forwardly opening chamber of the barrel of a lock assembly. A plug assembly that has been inserted to the operating position is releasably retained within the enshrouding barrel by a retainer that is moved by spring bias to a plug-retaining position wherein the retainer bridges complementary formations that are defined by the barrel and by the inserted plug assembly. A retained plug assembly that has been inserted to the operating position can be rotated about the central axis relative to the barrel between at least a pair of angularly spaced positions. One of the plug assemblies carries tumblers that are engageable with interior portions of the barrel for locking the one plug assembly in at least one of the angularly spaced positions unless the one plug assembly is unlocked by a suitably configured key. The other of the substitutable plug assemblies preferably includes a manually operable formation such as a knob.