Since Muncie parts can be interchanged during a rebuild the numbers may not tell you which trans it is. The best way to see what you have is to open the case and count the teeth on the input shaft gear.
M20 will have 24 teeth (less on some models)
M21 has 26 teeth on about a 45 deg angle
M22 also has 26 teeth but will have almost straight cut teeth (about 20 deg. angle)
here are two web sites with alot of Muncie info
http://www.corvettemagazine.com/proo...1/munciep4.asp
http://www.5speeds.com/muncie.htm