Somewhere in that thread (post-73) might be the Western Flyer serial number chart for the bikes that Murray built for their Western Auto stores.
I believe Murray started in 1936, which would correspond to the letter "A" - but maybe Western Auto did not order any bikes in that start-up year.
Most associate that early alphabet code as generally applicable to
all of Murray's bicycles,
not just Western Flyers.
For Sears, the early Murray-built bikes appear
not to be stamped M.O. for Murray Ohio; but are stamped with an "S" (for Sears). I have seen some 1938 SC bikes and 1939 SD bikes, but also a couple of S- Sears Elgin bikes by Murray. For those, no "A" and no "B" - I estimate 1937, (just so that I don't have to go thru the 17-pages again!).
Westfield also built Sears Elgin bikes in this timeframe, and Westfield's A=1936 alphabet code generally matches Murray's A=1936. Not sure if Sears imposed the alphabet code upon Murray (after that first year of S-only codes), to match their other supplier, Westfield. Could have been purely coincidental though.
The very first post in the Murray SN thread shows a later alphabet code which may reflect a post-war restart, and some overlapping sequences.