thats actually quite an old H50 that has place for the mechanical fuel pump so I would say sixties also there is no starter bolt holes that was an engine used for mini bike go karts or that reel mower
@7572, I looked up pics of my Snowbird Snowblower which also has a H50. my serial # is 4244. Being i know mine is from the 70's, Looks like yours is '74 then.