Do any millstones have concentric circles? The few I've seen had tangentially radial grooves, to lead the flour out to the sides. It seems to me a set of concentric ridges would not mill very well. But I'm not the expert, there.

It's also true I can't tell the exact shape of the whole--elliptical or circular--from the fragment. And that we're assuming the "whole" was symmetrically round. It might have been a meander, etc.