Abstract: A retainer cylinder is rotatably mounted in the bore of a housing cylinder and selectively latched against rotation by means of a resilient latching assembly. The housing cylinder is formed with a longitudinally extending groove and a corresponding recess is formed in the retainer cylinder. In one arrangement, the resilient latching assembly comprises a flat spring captured in the housing cylinder groove and having an intermediate portion resiliently urged into the retainer cylinder recess. A dog slidably mounted in the recess is cammed by a key to force the spring element from the recess and allow rotation of the retainer cylinder. In a second arrangement, the same housing cylinder allows for handle operation of a retainer cylinder wherein the latching assembly comprises a cam pin resiliently mounted in the retainer cylinder recess.