As of yet i have not found anything resembling the ring pattern all the belt drives are flat the weighted tire could be a possibility that was a good thought. But the ring still throws me off
When you're pouring concrete you need a form. If you have a pair of old wheel rims kicking around from a abandoned planter or bailer - why not put them to use? Cut away the hub, lay it on plywood and you have a form. Use a waxed bucket full of rocks (you can pull out when hardened up) for the center hole. Keep the edge of the concrete from chipping, too. Farmers did with what they had - some still do.
Making mill for animal feed out of repurposed material certainly is plausible on a old dairy farm I think the mystery is solved looking forward to posting more things as they get rediscovered