Hi zpi28,
My first impression was farm equipment wheel, too. Maybe John Deere 'D' tractor. I will go out and have a look 'round my stash, and get back to you.
L.B. MacAlister. [farmer]
Hi again zpi28,
I had no John Deere 'D' steel disc wheels, only spoked ones, but I still believe the 1947-53 'D's would have those 18" slightly concave wheels. Here's a Massey-Ferguson 95 wheel, 18" and six bolt, but convex.