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I have many WW2 Era Columbias and I will post the serial numbers once I have the time to get a look at all of them.
I've just gone through most of my late 30's 40's bikes both civilian and military and here are what I have;
1939 Ladies..........................................................................................Serial number D21134..................code E8
1942 Lightweight Civilian Compax prewar frame style.........................Serial number G29121..................code J1
1944 Marine issue Men's....................................................................Serial number MC14643A.............code L1
1944 Ladies Straight bar Civilian Blackout...........................................Serial number W76814A................code L9
1945 Military Model lightweight Compax (known navy issue).......Serial number J33428....................code 2N
No bikes in the 40's from 1946 on, of which I have 3 Compax have the code. The serial numbers although still under the crank hanger are obviously stamped with a different machine in a smaller, more uniform font.
The 40's data would seem to confirm you idea that in the 40's anyway these codes do indeed indicate year and month. An anomaly in my list is the 45 Navy. Could the N stand for Navy? The letter and number are in the opposite order as well. Is there any other known Navy bikes out there to confirm this?
Going back to the 30's for reference I have the 39 with an E code. This does not fit the code if counting backwards with J=42 then (I-41, H=40, G=39). The 39 is two letters off. Once I can get at another 39 of mine and a 41 I will confirm those codes and see what I can come up with.
In all I am becoming convinced these codes are year/month for wartime Westfield's anyway. Great work Mercian!