I found a very helpful site that decodes the Muncie transmissions by serial number and case number to tell you the ratio and DOM.
I thought my transmission was an M20, but turns out it's an M21. The 68-74 Muncie's for example have a serial number stamped in the casing starting with "P". In my case P9B26B 9 indicates it is a 69 year, "B" February, 26 is February 26 and the B at the end indicates it's a M21. The article explains that if it ends in "A" it's an M20, "B" M21, "C" M22. It's a good article and worth a read.
It was much easier to stockpile Muncie spare parts and cores in the 1970s than it is today. Reliable sources to validate information regarding serial and casting numbers will get scarcer as time passes. The specialty rebuilders that have seen hundreds, if not thousands, of these transmissions...