Chris I apologize for my poor photos but I am a tractor/motorcycle mechanic and not a photographer.
OK - more about the cylinder. If you are an astute knucklehead kinda guy and see this photo you will come to the same conclusion that Chris came to - 4 fins = 36 to 39. If you even more knowledgeable you will recognize this as a 1936/37 cylinder by the raised casting numbers. Somewhere around the 1937/38 split they changed the casting numbers to recessed - I have never been able to determine when.
Now - would anybody believe that this cylinder was actually made much later than that - 1944 to be exact - June of 1944. If you do not believe me go back to original post and follow the link now and see the second shot of this cylinder with its' 6-4 casting date. This is the original rear cylinder on my 1944 E knucklehead. My 1943 E has the same rear cylinder only with a 10-2 casting date. Ronnie Cox's 44 E has the same situation.
Here is the only explanation that I can come up with. All 3 of these bikes have a late 39 casting date on the front cylinder so I surmise that they must have had a bunch of fronts in warehouse and made a rear to match but somehow used the earliest casting mold that they had.
I would love to hear anything anybody has to say about this.