Did you check this thread?
https://www.retrobike.co.uk/threads/middleburn-crank-history.75203/
Not as straightforward as a simple timeline, but a lot of good info on there.
It's probably less confusing for people if you use GT's sizing, which was measured C-C. If I understand correctly, you are looking for an 18" GT frame, which measure about 20.5" C-T.
From my discussions with Sam, it seems much more convincing that the letters were months, but there are letters which were skipped. that certainly matches with the Hodaka GT serials I have.