The PTO was I understand made for Rover, who specified it , by Aeroparts in Hereford . The same company who made the winches & other PTO parts. Lots of the parts are standard Rover. For instance the idler is made from the same steel blank as the idler in the transfer box, but only machined for one gear . Regarding shimming, the parts book lists 6 gaskets, presumably you use as many as required for correct mesh. I fitted the bottom PTO to an 88" S3 back in 1980, so some time ago . I made a bash plate for under the bottom parts on the PTO as the pump fittings were very near or slightly below the bottom chassis rail . For info it wont fit on a standard FC as the transfer gears are in a different position, having constant mesh gears on both High & Low box. When I worked at LR I was able to view all these parts drawing at the Rover drawing office store
See attached extract from parts book