This is a unique housing tandem roller assembly often found on Andersen patio sliding doors as well as a variety of other manufacturers of wood sliding patio doors. It has two 1-1/4 in. concave steel ball bearing rollers set in a stamped steel housing with side adjustment. The housing is 11/16 in. wide, 1-3/16 in. tall and 4-5/8 in. long with 1 mounting tab at both ends of the housing. When the housing is inserted into the mortised area in the bottom rail, screws are used to fasten it in place.