Brand: Fuel Safe Fuel Cells
This D-shaped rubber flapper valve is used on most Fuel Safe 6x10 Standard Fill Plates (with the capped filler neck welded to the fill plate). It is held to the underside of the fill plate by 2 bolts (not included). When you fill the cell, the rubber flap pushes out of the way to allow fuel in. In the event of a rollover, the rubber flap closes off the filler to minimize spilled fuel.
Rubber hardens with age, which can reduce the effectiveness of the flapper. It pays to check the condition of your flapper valve at least once a season.
Remote fill valves use a different round flapper held in place with a single center bolt.