Patent ID: 7073360

Claim:
A lock and key combination for providing master key capability comprising: a) a master key having a first master key bit, a master key stop and a master key stop bit extending along an axis of the master key from the master key stop to a first end of the master key, said master key stop having a depth equal to the difference between the distance from the top of the master key stop bit to the axis of the master key and the distance from the top of the master key stop to the axis of the master key; b) a low level security key having a first low level security key bit, a low level security key stop and a low level security key stop bit extending along an axis of the low level security key from the low level security key stop to a first end of the low level security key, said low level security key stop having a depth equal to the difference between the distance from the top of the low level security key stop bit to the axis of the low level security key and the distance from the top of the low level security key stop to the axis of the low level security key, wherein the depth of the low level security key stop is less than the depth of the master key stop; c) a lock cylinder comprising a lock shell and a lock plug, said lock plug being rotatable inside of and relative to said lock shell, and said lock plug being configured for insertion of said master key and said low level security key; and d) a master annular ring, said master annular ring being provided on the lock plug and being nonrotatable relative to the lock plug and said master annular ring being in the form of a ring and being provided with a tab extending into the interior of the ring formed by the master annular ring, said tab permitting insertion of the master key stop bit into the lock plug until the master key stop contacts the tab on the master annular ring and thus permitting rotation of the lock plug from a locked state to an unlocked state, and said tab extending toward the interior of the ring formed by the master annular ring to such an extent that the tab permits insertion of the master key into the lock plug and said tab prevents insertion of the low level security key into the lock plug due to interference of the tab with the low level security key stop bit.