That's the inside of the 67-up pop open gas cap.
Underneath this is a screw and washer which allows one to remove everything from the swing-away cap and have it replated.
Pulling this off without destroying it is tricky.
I just did this in my build thread.
Another way of removing these is to sharpen a very small screw driver and pry the lips inward. It usually just comes out easily after the radial pressure is released. Afterwards find a thick piece of DOM (2"?), salami cut it and turn it into a curved anvil to straighten the edges. Then plate. Other ideas on removing are in the same portion of my build thread.
Spares are only available via others' gas caps. I supposed one could could just find a crappy spare pop-open top, cut the spring and standoff on the chrome side and get a punch in from the backside to remove it.