Patent ID: 8276416

Claim:
A master key lock comprising: a lock cylinder having a keyway for receiving a key; a circumferential sleeve, the circumferential sleeve being axially movable along the lock cylinder and the circumferential sleeve further comprising a plurality of transverse slots; and wherein the lock further comprises a plurality of wafer tumblers comprising unlocking/locking tumblers, a plurality of master tumblers and a drive tumbler, the drive tumbler having a window and a protrusion into the window and including a master key comprising a portion that contacts the protrusion to drop the drive tumbler, the wafer tumblers are transversely movable to engage the sleeve slots wherein insertion of the master key moves the sleeve and realigns the wafer tumblers relative to the sleeve slots about the lock cylinder; means for preventing circumferential rotation of the sleeve about the cylinder; a shim; and the master key comprising a point for contacting the shim, the shim contacting the sleeve upon insertion of the master key for moving the sleeve and unlocking the lock when the master key is inserted into the keyway.