Welcome Northerner

I wouldn't rub the stamp at all either - just leave it as it is.
From the scan you posted, I would be inclined to say that it is a 'possible' plate variety. Failing that, it might just be a 'fly speck' variety caused by dirt or some other foreign object on the plate while being printed. At any rate, it is well worth keeping.
On another note, if it was a piece of foreign object on the printing plate, the question would then be, how many runs were made at the printing presses before this 'object' was noticed and removed? This could put the variety into a 'constant' variety but I am more inclined to think that these 'constant' varieties are those retouches by the engravers / printers to the plate itself.
Chimo
Bujutsu