Need information on fel-pro gasket set?

I am rebuilding a SBC 350 and I have a Fel-Pro gasket set # ks2600. I know that I don't need all the gaskets they include but there is one I'm not sure of. It looks like a nylon flat washer. Reading the contents list it say's that there is a oil pump shaft retainer. The oil pump I have came with two plastic sleeves that I know lock the shaft together. What would this plastic washer be used for? Need to know before I get ahead of myself. Perhaps I just toss it over my shoulder I don't know. Hate to do that though. The list on the carton doesn't quite correspond with the contents. Say's it has an oil drain plug seal but not in there unless this part that I'm questioning is for the drain plug. Never seen a plastic one. This little part is holding me up on getting this bad boy fired up.