It is hard to find photos online that show detail of tail lights. I looked at what I could find of the Electra and it looked like that fairing had a rib down the center. Of course that was of a '63 (with this Lucas style lense fwiw) and I have no way of knowing first hand if different years might have had different fairings or for that matter if the photo I was looking at was original.
I looked for a variety of different Nortons from that period and didn't find any that looked right. So, assuming it is British, it could be from any of the makes, or it might have been an aftermarket piece. The thing looks new except it has a broken off screw in the threads of one of the mounting tabs. So for all I know some chopper builder back in the 70s broke the screw off on a new piece, tossed it on a shelf and it sat there until I came along.
Russ