Rigid handcuffs. Story goes that they were invented by a Dr. Lilly during or just before the civil war to be used on mentally ill patients to prevent them doing damage to themselves. It is further noted that Dr. Lilly suffered from mental disease later in life and spent some time wearing these himself. Finally, it is noted and historical fact that the Lincoln Assassin conspirators were placed in Lilly Handcuffs while on trial. They are rather restrictive and hard on the body, especially over an extended period. One of the more torturous restraining devices designed and used in the USA over the years. The locking mechanism is a bolt with a square head that screws into a nut on the other side of the cuff. A key with a square hold is used to lock and unlock them.