These are the "famous" McKenzie Mitts invented in the mid 1800s by Jacob Mackenzie. Possibly the first "high security" cuff ever constructed. The designer was a escort guard who was jumped by a prisoner and shot with his own gun during the escape. His response was to design this device.
The Mitts lock around the hands and are joined by a short chain which can further be locked to a Belly Chain. The idea was to prevent a prisoner from using his hands to gain access to keys and/or weapons from the guard.
Ultimately, it proved to be too high security by limiting the prisoner's ability to use his hands to such a degree as to prevent basic bodily hygiene (re: going to the bathroom) while in transit. Since a guard wasn't allowed to remove restraints from an escorted prisoner by railroad rules (standard means of transporting in bygone days) the task fell to guard to assist the prisoner in performing these acts. The guards were, understandably, opposed to this and the device never caught on.
The Mitts are applied the each hand. There are two locks per Mitt. The first locks the Mitt on to the hand, the second is at the end of a fine chain which is used to further reduce the ability to slip out of the Mitts by locking around the wrist. The same key works on all the locks, but it is nearly impossible for someone wearing these to use this key even if it were in their possession.