<p>A key operated rotary plug cylinder lock having rotary disc tumblers with a concentric cylindrical boss on one face of each disc tumbler projecting axially into internesting relation into a complemental cylindrical well or recess in the confronting face of the next adjacent disc tumbler. A lock-out pin mechanism to lock the plug against rotation when excessive rearward stress is exerted thereon, and a frangible lost motion coupling between the tumblers and the shell portion of the plug breakable upon excess torquing forces are also provided.</p>
