Chicago Handcuff Company Model X55 3-Way Handcuffs. The three cuffs allow one to handcuff a prisoner to someone else or to a stationary object. Center ring has a 2" diameter. NIJ approved handcuffs. 2 keys included. Weigh 15 ounces.
3-way handcuffs were more popular in the past. Most companies stopped making them by the 1940 and 50s. These are the only 3-ways that use standard handcuffs under current production.
Available here: http://www.handcuffwarehouse.com/chmox553ha.html