Abstract: A lock with a plug (2) removable from a housing (16) is disclosed. The plug (2) includes a restraining member (46) and a protrusion (40) which engages a concentric groove (52) formed onto a plug housing. Intersecting the concentric groove is a longitudinal groove (50). Upon inserting an operation key the restraining member is forced into engagement with the concentric groove thereby preventing plug removal even upon alignment of the protrusion with the longitudinal groove (50). Upon inserting a removal key (42) the restraining member (46) is disengaged from the concentric groove so that the plug may be withdrawn upon rotationally aligning the protrusion (40) with the longitudinal groove (50). In this manner a simple and compact removable plug is provided to aid rapid lock re-keying.