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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.
 
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