There was an episode of American Pickers and they had to find one of those to landscape the yard of William Shatner. Pretty effective way of milling grain and ore for thousands of years water wheel
I can't validate this information, but an old timer told me that the pre civil war mills had stones that were thick like yours. Then after the war the mill stones were only about half as wide as yours.
Back in the 70's when was 14 I found 3 on a wooded hillside . one was complete - half way down the hill - the other 2 were still un finished embedded the granite boulders .