^interersting to see that the z4m has a larger/different clutchpack in the diff than the m3 and that its much wider.
Did you open up the diff itself to see if there were changes to the actuator mechanism?
There's very little to see through the hole of the m3 diff, it looks like they may have swapped the location of the clutchpack from left to right. That might also account for some of the difference in size btw.
The interesting thing is, that in this thread (sorry, its in romanian), a guy is/was selling an m3 diff (or so he says) but its clearly one that looks like the z4m one:
https://forum.4tuning.ro/641-piese-b...sd-m3-e46.html
So maybe also the m3 was delivered with different diffs?
Maybe they changed the design during their production?
There is a p/n change on the m3 diff in 2002, so thats in the middle of production.
Its still not the same number as the z4 has, but that can also be due to the rear cover or so.
The p/n change from the z4m diff in 2014 is of course not a redesign (as the z4m was out of production) but maybe a new batch made or so to ensure parts availability.