they transitioned during 1974, I was looking at that engine this morning, Its possible that is an early HS50 internally. A-its actually stamped 5HP on the block, B-it has the aluminum flywheel that Tecumseh used for the first year or so on the first HS50's that seemed to be gone by the time HS40's and small frame H25-35's made the transition to the larger flywheels during '74....they went steel wheel with plastic fins at that point.