Rebuilding top end and valve train, got a bag of oil pump o-rings from HD store, there's one that's black and about 3/8" dia. that I can't find a place for in the shop manual or on the bike. -Input please?

- called up the parts dept. at local HD store, wanted to switch me to service and gave me attitude when I asked him to just look at his parts book exploded view, but he did so. Turns out there's a clean out plug at the top of the cam plate and that's where the o-ring goes.