I just finished changing the lift pump and fuel filter, but as I was priming the pump (the TRUCK'S pump) I looked down and saw an O-ring sitting on the inner fender. It's 5/8" OD, 3/8" ID, and I have no idea where it came from.
I looked through the FSM and don't see any possibilities, either. I don't want to start the truck until I know for sure that it didn't just end up there during another project... .
Any ideas?

Any ideas?