Page created 2005:
The M20/21 and M22 are for both 6 and V8. The six has a shorter input shaft than the V8's and a M21 6cyl box is pretty rare. On the end of the input shaft spline,an M20 has a single ring (looks like a circlip groove)machined into it.A M21 has two and a M22(tunna box)is bare. There were some M20 aftermarket inputs with no machining rings.
The Trimatic,Saginaw M21/M22, Aussie M15 & the Powerglide,M20/21/22, TH350, TH700R4, 4L60 & 4L60E all share the same slip yoke dimensions & splines, but may vary in universal size. The M76 Borg-Warner 5-speed found in VKs uses a yoke of a smaller diameter.