The M20/21 and M22:
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.