They are not super rare, but not one of the most common to show up. This is what the key looks like:
(not my picture, random find on google)
The nub at the bottom serves to keep the key in place so it doesn't drift in the keyway here there and everywhere. The next part up that sticks out from both sides is what interacts with the double and single locks. The chunky part at the top, if I recall, helps to also keep the key nicely aligned in the cuff but the important part are that upside down T shape with the nub sticking out the middle. I made my key out of a metal ruler.