1,235,717. Cylinder locks. CHICAGO LOCK CO. 7 Oct., 1968, No. 47498/68. Heading E2A. A key-operated cylinder lock and a key 12 therefor has a cylinder 14, Fig. 1 which is formed in a front end face thereof with an annular recess having a cylindrical side wall 21b, a generally cylindrical plug body 22 rotatable within the cylinder between locking and unlocking positions and having formed therein a keyway 24, said plug body being provided at its front end portion with an enlarged annular rim flange 20 seated within and substantially filling said recess, a set of tumblers 26 carried by said plug body and individually shiftable endwise therein between cylinder engaging- and cylinder releasing positions, spring means 30 normally urging said tumblers to their cylinder engaging positions, said tumblers having portions thereof exposed in said keyway for co-operation with a contoured surface of the key whereby upon projection of the key into the keyway to the fullest extent to which it is capable of being received, the tumblers are moved to their cylinder releasing positions, the forward region of said plug body being formed with a radial bore 50 which projects through said enlarged annular rim flange, said radial bore communicating at its inner end with said keyway, and communicating at its outer end with the annular recess, said cylindrical side wall of the recess being formed with a pair of circumferentially displaced internal notches 62, 64, (Fig. 4) one notch registering with the radial bore in a locking position of the plug body and the other notch registering with the radial bore in an unlocking position of the plug body, an auxiliary one piece tumbler 52 slidable in said radial bore between the inner and outer positions when the bore is in register with either of said notches, said auxiliary tumbler when in the outer position having its outer portion disposed in either of said notches to lock the plug body against rotation and having its inner end portion withdrawn from the keyway, and when in its inner position having its inner portion projecting into the keyway and its outer portion withdrawn from either of said notches, the key being formed with a socket 76 in a side portion thereof and in register with said bore when the key is fully inserted into the keyway, said socket affording a clearance region for movement of the auxiliary tumbler to its inner position, the portion of the key extending from said socket to the extreme distal end of the key being engageable with the inner end of said auxiliary tumbler and serving to prevent movement of the same to its inner position, each notch being provided with a concave surface which makes camming engagement with the outer end of the auxiliary tumbler to force the latter out of the notch and to its inner position when turning torque is applied to the plug body by the key. In the construction shown the plug body carries a turnbutton 18 for engagement with a suitable keeper.