This is certainly a unique restraint based on an Italian design which was still in use in the 60s.  Mediaeval in origin this design got its name because dungeons used to get flooded and the prisoners were said to be having a bath.  Bagno is Italian for bath. 

The prisoner's hands are put through each opening and the screw is then tightened.  A padlock is then applied (not supplied) making it impossible to unscrew the restraint.

The padlock is not really necessary as it is virtually impossible for the prisoner to reach the nut when he/she is secured.