Odd. I've only ever seen it on 1970 cars unless put there by the owner.
Maybe some dealers used those in 71?
someone with more history on it can answer?
Selling a '71 ECS ('70 CA) gas cap, filler neck, and vapor canister. All are in very good shape with no rust issues. The gas cap could be sanded and repainted. Asking $200 shipped for everything.