A thread on another forum not a million miles away is asking this question.
The answer is GLOSS.
The black stoved enamel on Nortons of this era was renowned for its deep lustrous gloss black enamel.
Stoved enamel - as in baked.And applied by dipping, mostly.
You can sometimes see the run and drip marks.
This is a sample of a 49 lower mudguard section - it you look closely the run and drip marks are still apparent.
Its also suffered a bit over the years, it would seem its not entirely scratch resistant.
You can tell its 1949 by the regulator piggybacked onto it - in later years it was inside the toolbox....
I'd further comment that Nortons stove enamel of this era generally wasn't too robust, you don't often see 'original paint' bikes,
the enamel seems to weather off rather well...
BritBikes seems rather flakey today, have to try everything 3 times...