SW date code chart in case anyone is interested.

Rick, I sent you an email that might help you with the reducer.

The only thing I will add is to rebuild it before install. Mine had very dry grease inside that was pretty much solid.
From the various photos I have seen, it seems like they went from Phillips head screws to hex around 1975 or so.

FWIW, the '70 Yenko Nova's used the round units with a .8333 ratio.

Based on all the information I have had sent to me, called to me, etc (and thanks to all who did that) I have come up with 6261794 or 3708144 (depending on plastic vs metal gear) drive gear which natural in finish and 8 tooth, 3987919 driven gear opaque and 19 tooth and 3932220 .7333 adapter. The kicker is that is what is shown for the F70x14 tire. I can find NO information on what may have come with the F70x15 tire combo.

Photo is from the Ed Cunneen Copo Connection Source Book

Thanks John but those gear part numbers are for a Turbo 400 not an M21 unfortunately.