None of my original gas caps has a decent gasket so I went looking for a vintage paintable cap or even a stainless one. I stumbled across some cross referenced vehicles, the easiest & most common being '38 to '70 Chevy trucks which take a vented cap.
Some of the NOS chevy caps I saw have a ring stamped into the dome of the cap but not the new one I bought.
I got a paintable one, GM#3914958 Not a perfect copy but pretty close.