These look sort of like Peerless Leg Irons except that they are much cruder in form and don't have swivels for the chain. The key LOOKS like a standard key but has a slightly longer bit. Using a "standard" key in the lock will cause the key to be trapped in the lock. There is no clean way to undo this so caution is advised! Double locking is accomplished by turning the key in the opposite direction to the unlocking direction. Once double locked, a full turn in the unlocking direction will double unlock it. These are blued but not prone to rusting.