Grist mill plates were not made from concrete, way too soft,
also they do not have any other holes other than the axel hole with assorted drive cuts or a square central
hole.
Plus they are cut on an conical angle for the ground product to come out,
the upper plate has a depression almost matching the bottom,
but with larger clearance in the center for the product to move into it.
The ones around here are made from granite, hard as hell.
There were a lot of water driven grist mills around here from the 18th and 19th century.