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Hello, I'm Tan. Welcome to JustAnswer. I'm reviewing your question and will reply momentarily.
Do you have a model number please? The number you posted doesn't help.
Ok I will be here. They make many different models all the elongated tells me is the size of the bowl.
Is this the flap you are referring to # 11? This is the joker valve. This allows the water/waste to flow in one direction.http://www.downwindmarine.com/images/P/91002557.jpg
Does this look like your head? http://www.downwindmarine.com/images/P/91002557.jpg
This is the head for the model number you posted. As you can see in the diagram there is no flap.
The valve I posted is the joker valve an it stops the waste from coming back in the head.
Does this look like your head?http://www.downwindmarine.com/images/P/91002557.jpg