Abstract: A security system having a secure enclosure for holding an object. Access to the enclosure is controlled by a combination lock having a tumbler arrangement adapted for having its combination changed either by hand or with use of a key. The tumbler arrangement includes an apparatus for mounting a plurality of tumbler wheels in a stacked relationship for rotation about an axis. Each of the tumbler wheels has an inner element releasably engaged with a circumferentially disposed outer element. Each of the outer elements has a peripheral gate and a mechanism for masking the gate. A mechanism is provided for maintaining the inner and outer elements in an engaged condition for simultaneous rotation under normal tumber operation, and a peripherally gated rotatable driver wheel rotates the tumbler wheels. A fence lever includes a nose for contact with the driver wheel and a fence for contact with the tumbler wheels.