See how many rings are on the input shaft.
2 Rings is M20
1 Ring is M21
0 Rings it may be a very rare 67 M22. Also would have a drain plug on the side bottom, where the others wouldn't.
If there are no rings on the input, then
open the side cover after you drain the trans.
See if the gears are cut at about a 45 degree angle or a 20 degree angle.
20 (or thereabouts)degree is M22. CHA CHING.
__________________
Rick
69 Chevelle SS396 Silver L78 M21 Post Sedan (Berger Chev.)
69 Chevelle SS396 Black L78 4-speed 4.10 Post Sedan
Don't text and drive.
Somebody's life may depend on it!