Thanks to 428Street I became aware of the different type fuel filler used
on the street Cobra's like the ERA I'm waiting for delivery of. The Norton Commando bike fuel caps are reproduced and fairly easy to find but I did have some problem finding the fuel neck the cap mounts on. I finally located a Bike parts company in England that had them and the caps. If anyone else is going through this and having problems finding a filler neck to use with the cap here is a source
http://www.burtonbikebits.net/petrol_tank_caps.htm
06-681B 68-8190 non-locking gas cap.
06-681A fuel tank neck.
Dan