This is an aluminum hinged cap from the beginning of the 1970s. In 1974 BMW chromed the gas tank caps. They were shinier, but did not retain the nice patina of the aluminum caps (instead chrome bubbles off).
I discovered motorcycles when I moved to Oakland. It seems that the Bay area is the best place on earth to meet Airheads enthusiasts.
When I'll go back to Europe at the end of my contract, my new old bike will be a keepsake.