A mortise lock includes a case having first and second sidewalls. A follower plate is pivotably mounted in the case and actuatable by a key-operable cylinder to move a latch bolt from an extended position to a retracted position. A locking plate is moveable by the cylinder through transmission by a lever link between an unlocking position allowing movement of the latch bolt and a locking position not allowing movement of the latch bolt. The locking plate is active when a connecting member is slideably extended through a slot in the first sidewall and through an engaging hole of the lever link into another engaging hole in the follower. The locking plat is inactive when the connecting member is slideably extended through another slot in the second sidewall and engaged in the engaging hole of the follower and not engaged in the engaging hole of the lever link.