Hi guys, I found an interesting topic on the Italian forum:
http://www.morinispecial.it/joomla/foru ... 5&id=15636
It's about the gear that drive the oilpumps on our 3 1/2's, 500's and 501's. Apparently when the company changed to the plain bearing on these engines they changed this gear to one with 7 'entries' instead of 6. It should give more pressure to the plain crankshaft bearing as where the rollertype doesn't need it this much. The oilpump remains the same on all models according to the link.
Might be worth considering if someone is rebuilding an engine from a pile of parts and doesn't want to get these two mixed up.