This is the famous Plug-8 handcuff manufactured by Froggatt. This cuff is difficult for the inexperienced to open, even by someone not wearing the handcuffs. First top end of the key is used to remove the plug from the lock case. Then the key is turned one direction to pass through the top of the entryway and the the opposite direction to actually open the cuffs. Both left and right handed versions of the key and mechanism can be found. These cuffs were made my several different manufacturers. This pair is marked Froggatt and was manufactured by the Thomas Froggatt Company.