As I recall, the 2-bolt backplate was only in production for the first year because it tended to leak. It was replaced with a 3-bolt backplate. It will definitely have a 4-bolt head. I got a 3-bolt backplate one of these back for Christmas 1951 so this one is probably a 1950 model. Mine, as I remember, was stamped with 35 on the side, but I might be wrong on that. Brings back the menories.