I have a 4 speed Muncie.I bought it as a M21, But i think its a M20. This is the info i got off of it. Front case side # 3851325, back case #gm3846429. Top case (maybey vin )?? 824p064921. Side case by the linkage,3831707. Input shaft splines 10. Output splines 27. No ring groves in the in put shaft. Help me figure it out. Thanks to all who help.

The 1325 case was used in 64-5, and has the smaller 7/8" cluster pin. If it's an M-20, it has the lower 2.56 1st.gear.
The only way you'll be able to determine if it's an M-20, or M-21, is to remove the side cover, and count the teeth on the input gear, or count the teeth on the cluster gear.
The gear on the input shaft should have 21 teeth for M-20, and 26 teeth for M-21.
Hope this helps.