Apparently, there were some 21" s produced without the bead edge. They had the rim edge same as 1930'31 19"'s.
A couple of FBers have posted pics of them . If you have a set, I would use them, as a talking point.
The so called AR wheel had a bead edge rim like all 21's, but the hub area was slightly different shaped to fit the early type drums, the rears without the separate brake band.