Also the early Muncie had the 27 spline output shaft the late Muncie had the 32 spline output shaft. Only way to be really sure it is an M22 is to pull the side cover off and check for the shallower angle cut gears. Most guys get fooled by M21's thinking they are M22's
You can buy brand new gears for the M22 at around 500.00. So just get on ebay and buy a good case with the 1" countergear shaft, tailshaft and sidecover. Then buy a new set of gears, bearings, syncro cones. And you should have yourself a good M22 gearbox.