Something doesn’t look right in your first photo as there is no compression relief at all on that cylinder head. The only thing I see is the hole on the rewind starter for the pivot pin for the valve linkage. Either the head has been changed over or the whole powerhead has been replaced at one time.
Look on the block for a welch plug with some numbers stamped on it. It’s about the size of a quarter and might be on the top of the block. The numbers should match the ID plates numbers which is mounted either on the front or the sides of the transom clamps.
Mumbles, I think that the cavity in the rewind starter is there for any of the big twin engines after 1957. I have a 1962 28hp that has the hole for the compression relief in the rewind, but the 28s and 33s didn’t have the compression relief. It is probably much cheaper to use 1 casting instead of 2.