The Yale Door Closer 51 Series has been around since the early 1970's. With the advent of the aluminum framed storefront doors, this door closer was a perfect fit. The 50 series is a non-sized, non-handed closer that can be mounted on the push (parallel arm), pull (regular arm) or top jamb. A Grade 1 closer certification, means it is one of the more heavy duty closers out there. Older models may be stamped with Scoville or Eaton, which are the companies that used to own Yale. We some times find 20-0054-4001 or 20-0254-4001 stamped on the back of the closer. Ships with all mounting screws and a parallel arm bracket. The hole spacing is 3/4" by 9-1/16".