This chain was used by the earlier "Carabinieri" (italian police). Normally the chain also has a lock with it, so you can lock it, if you wrap them around a person. It's easy to carry with you and also fits every size of hands. You put the chain through the rings, so each hand has their own "cuff".
This handcuff was used until the 70s. Most likely, the name came from the non-official name of the prison in Marseille (French: "bagne" for bathroom). Thats because the prison was flooded often. You use it by screwing down the middle bar, until the person can't pull out their hands anymore. To close it so you can't screw it open again, you use a lock.