M20 or M21

[ericisback]

I think I know the answer to this question, but never hurts to ask.  Is there anyway to know if this is an M20 or an M21 from the attached photo?

[WorkinProgress]

Is there any ring grooves on the input shaft?

                          - Warren

[z28z11]

You can see one groove, there is a suggestion of a second groove (which 2 would denote an M20), but it's not clear. If the pic was shifted forward about an inch you could likely see for certain. You know it's a 1" cluster pin Muncie, '68 likely.

Doesn't appear to have a suffix on the build code, looks like an "8" to me. What an eye - stretching exercise -

Better pic of the input and you can figure this one -

Regards,
Steve

[ericisback]

Is there any ring grooves on the input shaft?

                          - Warren

I only have the photo.