Well done!
Midland used M as a code later in the war on their AT-11's. I have some base books with several aircraft showing M codes Here is a sample where you can see M123 painted on the belly

I have an AT-11 that was at Midland and it had 1D83 on the side according to a photo in an accident report. I wonder if it became M83 later.

I also wonder if the 0 and 1 prefix codes were somehow linked to a flight within a squadron. I wonder about the D significance too. I also have Midland class books with the following codes on AT-11's:
1D29
OD14
OD66
OD6
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