Check my post below titled: "Engine Serial Number Help", a few of those notes should still be helpful. Also will attach a few photos to help ID the different manifolds. These are not the best angles for viewing but all I could find at the moment, so rather than looking for the curve of the runners, in these views note the large square casting connecting the intake and exhaust runners on the early manifold vs the 3 thin ribs on the later manifold. Also note the distance between the lower runner and the carb on the two manifolds, the later manifold looks like the carb is much closer.
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