This is a plain back tandem roller assembly with a 1/4-20 threaded mounting hole and two narrow mounting slots on the back end. These slots slide onto an opposite shaped extrusion on the door frame. The adjustable stamped steel housing is 11/16 in. wide, 1-1/16 in. tall and 3-1/8 in. long. There are two 1-1/4 in. concave or grooved steel ball bearing rollers. Used by a variety of sliding patio door manufacturers.