Bore and stroke differences were my thought too. I also would have expected the carburetor throat to be larger on the K-50.
Thanks for confirming that I wasn’t overlooking something obvious. We are dealing with merely 90 year old outboards here, who knows what parts have been repclaced over the years to keep them running (I was thinking that may be the case with the K flywheel on the A motor).
I guess I’ll have to tear into them and report back my findings to help the next guy who finds himself asking this question.