The original John Deere pitman less sickle mower was the Model 10 side mount, introduced with the New Generation tractors. The knife drive design was not adaptable to the 3 point or pull type mowers, so the belt driven 37 pull type, 38 rear semi mount and 39 3 point we’re all pitman drive mowers. In 1965 the Model 50 side mount was designed to fit the new utility 1020 and 2020 tractors and was also adaptable to all Waterloo row crop tractors, so the Model 10 was history. The knife drive of the Model 50 was adaptable and was used for the 350 3 point, 450 drawn and 250 side mount mowers, introduced with the Generation II tractors. There was never a rear semi mount pitman less mower. Hope this helps.