There are some differences between the early 1st design muncie and the later
2nd design muncie, One difference is early and late syncro rings and syncro hubs
you can use either one, but you must use the correct ring with the correct hub.
the picture below shows you where to measure across the hub the early hub measures
1 1/8" across and the late hub is 1" across. now look at the syncro ring picture
look at the top part of the ring on the left side,directly behind the pointed
engaging teeth the ledge of the ring comes out about an 1/8" then it steps down
and comes back another 3/16" or so,this is the late ring,it is used with the
narrower 1" hub. the ring on the right does not have this stepdown and is
used with the wider 1 1/8" hub. Also i get asked alot how do you tell the
difference between the (M20,M21) and the M22 transmission, you can not really tell
by looking at the case ,the difference is the actual cut of the gears,you can see this
in the picture below, the M20 and M21 have a much greater angle to the teeth of
the gear and also will have a twist in the teeth (helical bend), the M22 gear is a
more straighter cut gear teeth and also thicker and wider gapped teeth very
noticeable in the picture.