I got this history info from Anglefire. The 500 was made 1951 or 1952
The 500 and 600 represent crankcase reinforcement added in the coil support area, and the 300xxxS and the 600xxxS engines had a kick starter. Practically all of the engine parts are interchangeable, and that makes for some strange combinations. there were about 50,000 combined 300, 300S, 500, 600S and 700 engines made, and if you add the 139,000 Hs and 50,000 Js a grand total of about 250,000 Whizzers. There are a surprising number of them Left.