Just like Hiatt handcuffs, the leg irons use multiple different threads depending mainly on when they were made. ie really early ones often have a thin key like a long handcuff key, modern ones have an alloy key, in between eras (I cannot put a date to) are more standard but not the same and then there are the oddball Hiatts. If you can get hold of an old key, you might get lucky and it will work, otherwise you'll have to get one made.
First thing you should do is see if they work.
Get 8 inches of thin wire,
bend it in half,
twist it and make a loop at the end,
slip the loop over the bolt inside,
twist the wire a bit more until it grips the thread,
pull on the bolt to release lock.
I've been able to determine the key requirements for Hiatts by pressing a soft object along the screw thread (to measure the thread pitch) and photographing the screw thread (to measure the thread outer diameter). Its a lot of bother, but worth it for rare items.