A key operated locking device (10) comprising a lock casing (12;14) and a lock-block (18) slidably supported between first (26) and second (28) walls. The lock-block (18) is displaceable between locked and unlocked positions by first and second keys (20;20′). A series of flat members (404-413) extending side-by-side are slidably positioned within the lock-block. Each flat member comprises a cut-out (404′-413′) through which the first and second keys (20; 20′) may pass. Series of first (101-113) and second (201-213) cavities are formed respectively in the first (26) and the second (28) walls. By passing the first key (20) through the cut-outs (404′-413′), the flat members (404-413) become cleared out of their respective cavities and the lock-block is enabled to slide into locked position by the first key. Releasing the lock-block is attained by the second key (20′).