Puzzler - Engine & Gearbox numbers

I've got a puzzler here and am reaching out to the old timers and obsessive Norton Owners.

I purchased an atlas barn find with some really odd engine and gearbox numbers.
engine - D6 - 7713
gearbox - NA 1938

With some minor research the best I can figure is this bike may have been bobbed in the late 60s or 70s.
Additional recent findings trace the bike possibly back to San Francisco.
Does anyone know of any British Bike Shops in San Francisco open in the 60s or 70s that would have restamped their rebuilt engines or gearboxes.
The engine is not typical. It looks like it was built for speed. It has a lot of period correct performance parts on and in it.

Thanks for your help!

Pictures ?
Some shops had quite distinctive looking mods.

As on another forum, that engine number is clearly not factory..
Gearbox number sounds in the ballpark, the Factory records may be able to attach it to a particular bike.