Manufactured and used during the 1960's. Their unusual shape notwithstanding, these are "regular" enough double locking swing-through handcuffs. The cuffs have a single swivel. The pawls have no groove. The key, however, is somewhat unusual. It looks like a simple tube with a small bite missing at the bottom. The 'bite' catches on a pin in the keyhole post which is actually part of the ratchet latch. Double locking is achieved by turning the key in the direction opposite to the unlocking direction.