Slave Leg Irons
These are typical of "classical" slave shackles. These sorts of shackles were in common use in the U.S. and elsewhere to keep slaves in place. They were simple in design and cheaply made.
One side is a "puzzle" shackle. It is placed around the ankle, and the chain and loop arrangement, once assembled and end locked with the other shackle (placed around the other ankle) do not come apart again until the other shackle is unlocked. The other shackle, in this case, uses a simple screw to bolt it shut. The key needed (not pictured) is a long cylinder with a square sided hole in one end and a handle on the other. The hole matches the square end of the bolt.