I just installed a set in my 53, My set came without a mount for the shoulder. I used a 3x3 3/16" steel plate. I drilled a 1/2" hole in the center and two 1/8" holes along one edge for attaching a fish wire. On the back I welded a 7/16 fine thread nut for the attachment bolt. After drilling through the body with a 1/2 drill I dropped the nut assembly down using the wire. A magnet will hold it flat while you start the bolt. I used a dab of silicone on the face of the steel to hold it in place if I ever remove the nut again. No problem with the nut spinning since I had th wire attached. After assembly was complete I jiggled the wire back and forth a bit and snapped it off.
