When I was in (Kyushu) Fukuoka Japan 1967-1969 the Japanese Police were using cuffs simulate to those with the holes in the single arm. I do not think that there was any double lock mechanism, each cuff was marked Fukuoka Ken (State) and each has a number stamped on the cuff, apparently they had a list of what cuff was issued to each officer.
The lock mechanism was totally different, it used what appeared to be a flat steel key and pin tumbler mechanism, which when the key was pushed into the keyhole on the side, not face of the cuff, a rectangular block of metal could slide into the cuff and release the star wheel, allowing the cuff to be removed. This lock was similar to what many gas stations in the 50 &60s used by the employees to operate the cash register, you only had to push the key into the keyway, the rectangular block would then be slid in and the cash register could be operated, it was only necessary to pull the key out to lock the register, no turning was required, the key was normally attached to the clerks with a string or leather thong fastened to their belt.
This was the only video I could find to show the type of push key operation, but is operated by continuing to push the key in as the lock was fixed and part of the cash register
Oh Yea, another thing, the key to the police handcuffs was located at the police station, the officer did not have one.
Very nice duty in japan, and the entire time I was there the exchange rate was 360 Yen to the Dollar, actually we used MPC for money, but that is a different story.
https://en.wikipedia.org/wiki/Military_ ... ertificate
Howy
Cincy