All of the oil bath hubs I have ever worked on had a rubber cap on the see through plastic face plate. And the place I always got the face plates and seals was at the Caldwell Kenworth dealer. They had every cover for every hub ever made. Oil bath that is. I would suggest getting at least two spares and always have them with you. There is a pin hole in them in the center. If that pin hole gets plugged up, the oil will heat up and expand. I have seen them blow out pretty good. If you can see the pin hole, make sure you keep that cleaned out before every trip. Do you have the name or model number? Might be a NAPA part.