It looks like the usual lousy fit in the hole for the button on the cap. Typical of getting eaten in the acid bath before chroming. That's why this fella came up with these
Gas Cap Release Button 289 Early 427 Street [289 Early 427 Street Cars] - $44.00 : Acton Custom Enterprises, Custom Metal for Cobras
It looks like the pin for it might be a little big. The Norton Commando is almost correct for later ones. They should have a smooth rectangular button like these.
Street Car Gas/Petrol Cap Street Car Gas/Petrol Cap [Street 427 Comp Dry Sump] - $72.95 : Acton Custom Enterprises, Custom Metal for Cobras
CCX FIA is neat, my friend around the corner has one of the six in a similar color.