Another old bump, but I think I got to the bottom of it. I just need confirmation as I may have made a pretty dumb mistake in thinking the two washers (number 21 in the first pic, and 4 in the second) were the same in these two pictures: https://rennlist.com/forums/8593802-post8.html

 

21 is a washer, 4 is the green o-ring. Put the oil pump drive gear in first, then the o-ring, then the washer. The cam belt drive gear goes next against the washer. The o-ring fits into a small grove in the cam gear and gets sandwiched in to seal against the crankshaft.