Grinding Wheel find?

[Diver1111]

I post these pictures hoping someone can tell me how this grinding? wheel was used, exactly. What throws me is the 1/4" deep groove in the centerline on the face of it. If it is in fact a grinding wheel, why the groove? It runs all the way around it, as if it was there to hold something.

Weight-about 40 pounds; Material-appears to be sandstone; Diameter-19 & 3/4"; Thickness-1 & 3/4";

And what's up with the striations? Those grooves on the side about 1/4" apart?

I found this in June of this year in a "New Hampshire" body of water-not Winni.

Any help appreciated.