It looks a little big for it, but I remember ages ago seeing something similar, which the museum claimed were “Paint Pots.” The Indians would put water in it, and rub it with a long thinner stone all around like using a mortal and pestle without anything to grind. The stones they chose would give off a fine powder which would mix with the water and produce paint they could use on their faces, or to mark stuff. The stones were weird, in that they could be brown, and produce vivid red paint, or be black and produce a white paint. The more they rubbed, the thicker the paint got.
It might explain the uneven wear pattern, as they would look for a good spot to rub, and wear it in over time. What is the stone like if you put a little water on it and rub it with another stone? Is it wear resistant, or does it wear finely and produce a paste?
It is wear resistant. Folks just don't understand what I've been writing. This stone is NOT limestone. It is VERY hard and I can see flint in a chipped off spot.