Hi Mike,
Attached is the 3 bolt divers helmet distributor to a crab distributor adaptor plate I mentioned.
Looking at it, and the description, a cam button spacer is needed for some stock shallow cam noses.
So perhaps the other way to use an earlier distributor on a post war engine and vice versa is to also change the timing gear cover. Again, I am not sure if this is possible so perhaps another person can enlighten us. I agree, I don't know why one would want to use a divers helmet distributor as opposed to a crab distributor on a later engine but nothing surprises me when it comes to the old engines!
So to get back to David's original question, once he inspects the water jacket openings in the block that will be the final arbiter of what approximate year his engine was manufactured. After that it will be easy to decide what, if any, items have been changed from original.
Cheers,
http://www.speedwaymotors.com/Flathe...olt,25076.html