Mark, Sorry I didn’t see your question before. I have had to replace two caps because the rubber got old and stiff. My mechanic found replacement caps at McFarlane Aviation. He said he will get the part number when he gets back to town. The caps he found required two minor modifications. First, the metal plate on the top has a larger diameter and had to be trimed on one side to fit the left outboard tank. It fits the other tanks with out triming. Second, the cap is vented and the fuel tanks are also vented. With a full tank the negative pressure over the top of the wing will suck a small amount of fuel out through the cap causing a fuel stain. My mechanic solved that problem by taking the cap apart and pluging the vent hole with a soft rivet.
I will try to send that part number later.