I've noticed this on several old lenses I have. There is a thick, black, porous-looking but hard "deposit" on a large fraction of black-lacquered (I assume not enamel) lens barrels. It looks like adhesive residue but doesn't soften with or remove with typical solvents like kerosene.
What to I have here, and how can I remove it to show the lovely shiny black finish which I assume still lurks underneath?
They are ill discoverers that think there is no land, when they can see nothing but sea.
-Francis Bacon
It looks like someone slapped on some flat black enamel paint where the lacquer had been damaged. I've got quite a few black lacquered lenses and haven't seen anything similar, although I've noticed that the finish can be fragile. Another possibility is the barrel came into contract with a solvent that damaged the finish.
A clue may come from looking to see whether it continues down behind the flange. If not, it may be paint from touching up (or painting in the first place) a lens board, with the lens in place.
I have some strobe barndoors which were pretty heavily decorated with ancient gaffer's tape; the adhesive residue seemed to be immune to everything but lacquer thinner, which also took off the paint it was sitting on. The solution turned out to be coating the residue with mechanic's hand cleaner ("Gojo") and letting it sit for a while. The residue then wiped off without damaging the paint.
The usual caveat applies: if you try this, test as inconspicuously as possible first.
Have you considered that the changed appearance may be due to aluminium corrosion underneath the enamel? Some makers weren't too lucky with their alloy mix and/or the cohesion of aluminium surface and the enamel (aka schneideritis).
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