Rear number plate

..about Norton fitment, but many bikes ran a 3-row number plates, ABC at the top, 123 in the middle and the year letter at the bottom. This might help you?
Certainly was common practice until the late '70s. Even the Met Police Triumph and BMWs used them to allow narrower fitting panniers.

Regards, George 

There is plenty of information regarding reg' plate sizes on the net spanning all of the different year requirements complete with number dimensions to suit.
Norton will have sized their number plates to comply with the regulations set at the time of manufacture.

The number plate would have been made by the dealer that sold the bike. Norton would not have made it, so it would be whatever size the dealer decided to fit. 

The plate with pressed letters was added by the dealer onto the factory plate which carried the rear light.  But the answer to your question is: yes they did change the factory fitted plate.  The earlier bikes had a 'closed' box shape.  Later bikes from about 1963 (when the 'A' suffix appeared in most counties) had an 'open' form which was slightly wider to allow for 4 letters side by side.  This is lighter with no sides and a tapering top facing forwards.  Replicas are available today.

You can get a wider bolt-on rectangular plate, or attach a three row plate to the earlier slightly narrower factory plate to carry suffix letter A, B, C etc) by itself at the bottom.   I think they look a bit odd because the plate hides the bottom of the rear mudguard.  I believe rectangular 3-row plates are still legal on historic machines but not allowed on new ones today.  Today's rules do allow slightly smaller letters on motorcycles than on cars.  Old ones can still be hand painted.