There are two washers, one aluminum (?) and other other copper, along with cartridge housing cap o-ring and the filter, in the MB oil filter "kit" for 617.951 engine.
The washers seems to fit perfectly around the rod that's attached to the housing cap and centers through the filter cartridge but I'm not noticing any that they're supposed to replace.
Definitely not for the drain plug or the two nuts for the housing cap.
I went ahead and slipped them onto the centering rod, with the copper one touching the top of the filter...just to put them somewhere.
Any clues ?
I hope I didn't miss something very obvious.