Your last set of numbers maybe 91203 or 9120b so that would be 1969 the 120th day of the year and a shift or line number. But you need to know what your looking at because it gets wacky. 69-79-89-99 but there are things you'll see on a engine and you can place it in the correct decade by looking at it. After '04 they changed it. But when you find a minibike specific H50 it should stand out. and I think they are all 68-70 or 71 but I could be mistaken.