The 51 loader was made for the smaller 655 tractor and was later replaced by the 60 loader. The 755's orginally had the 52 loader which was replaced in the late 80's by the 70 loader. The 60 and 70 loaders are quick attach but I'm not sure if the 51/52 were.
I see there is 51 loader up on Ebay (from McGrew). They say it should fit on a 755 but I don't think it will. The 655 had a much smaller frame compared to the 755. You should really ask a dealer to confirm.