Newbie here and a prospective buyer. I now own Harleys and I want to add some Brit MC's to my garage. I'm looking at a 1973 decals say 750 Commando. The present owner claims the engine is an 850. If the engines differ, what distinguishing physical differences are there? Does any part of the VIN depict what engine it is or supposed to be? Any help would be greatly appreciated. Thanks in advance, Jim.