It's a lid I bent up out of sheet metal to fit a homemade steel casserole pan that I built. It also works on the #9 Griswold we were making the pizza on.
Ah! I found an aluminium lid at Wal-Mart that fits my pizza pan but thought maybe you had found something made for pizzas. I saw a home altered dutch oven that was turned into a pizza pan. Ill try to find a picture of it again. It was a guy on YouTube that had it.