Actually, pushing in the in the liitle peg in that hole
enables the double lock. This for the peg on the handle end of the key to fit in.
Turning the key in the one direction unlocks the
double lock , then the other direction unlocks the cuffs.
As long as the peg in the hole is not pushed in, they are only single locked.
All my cuffs and leg irons ect. work this way.
On a side note, you can use the springs that go under
batteries in old radios, flashlights and such (if they are no longer working)
and straighten them out with a pair of pliers to make very strong wire tools for
picking cuffs and other lever type locks.

What is the best way to educate the masses? ... " A television in every home."
What is the best way to control the masses? ... " A television in every room."
From "Charlie" AKA " Flowers for Algernon"