any help decoding these numbers on my tranny would be appreciated. 3925660 (GM) and the tail numbers are 3857584.

Those are casting numbers for a Muncie M20, M21 and M22. They started using those castings in 68 according to Alan Colvin. Same casting numbers were also used in 69 but I don't know if they were used any later (My Chevy by the numbers only goes to 69)

Hopes this helps.

Victor