Welcome to the WFM forums and to a group of information-sharing friendly folks, some of whom have had or still have one or more 322 tractors -- like myself.
The rear PTO on the 322/318/332/420/430 tractors came in various versions...usually when someone mentions the rear PTO they are referring to the Gear reduction 2000 RPM PTO and electric rear clutch/brake combination. Another variant is the 'rear-drive' unit for the 30 Hydraulic tiller (which uses the same electric clutch/brake, but a dual belt drive to the hydraulic pump at a somewhat higher RPM.) A third variant is the MCS that used a single belt drive, but again the same electric clutch/brake assembly. The driven shaft of the PTO clutch/brake assembly is a bit different in each case -- but any and all of them fit any of the tractors listed above.
Here is a quick exploded view of the kind for the tiller...
30 tiller rear PTO.pdf
And here is what one looks like off the tractor:
If you have the 2000 RPM gear reduction version, there is a gearbox that mounts behind the transmission housing as well, instead of that large flat black plate and the double sheave pulley. Are you selling your PTO, or planning on moving it to a larger tractor in the near future?