Oh man.. This is a stretch, but as here's a copy of some old hand written notes..

Muncie M-20 Wide Ratio No groove 2.56, 1.91, 1.48, 1.00
Muncie M-21 Close Ratio 1 groove 2.20, 1.64, 1.28, 1.00
Muncie M-22 Rock Crusher 1 groove 2.20, 1.64, 1.28, 1.00
Muncie M-20 / M-21 Special Order 2 grooves 2.52, 1.88, 1.46, 1.00

I'm fairly sure the wide ratio (M-20) did not have a groove on the input shaft in it's stock form, but they have been known to be replaced.

I think the "Special Order" were available in early (1963-65 Biscayne with the 409 only?)

The M-22 is easily identified once you open is as the gear teeth are beefier. The M-22 was only produced in a close-ratio version. If anyone says they have a wide ration rock crusher, they are "mistaken".

The input shaft and cluster gear are different on the M-20 and M-21.

The input shaft, cluster gear, 1st, 2nd, 3rd and reverse idler gear are different between the M-20/21 and the M-22.

Everything else will interchange. Synchro rings from newer years will work in older years but not the other way around so they should be changed as a package.

Good thing I wrote this stuff down - I'd never have remembered it! I think I kinda smell some heavy gear oil?

Regards All,
Glenn