I think the name of the seat is Easy Rider or E Z Rider. Don't know if Monroe made them or not, but they made after market seats for many different tractors. It was a very popular seat on H and M Farmalls. We had them on Oliver 70s. I think you could get them for some of the old John Deere A and B models. We had neighbors that had AC WD-45s with a version of that seat, don't know if it was standard or not.
It was a very comfortable seat commpared to the factory seats of the time.
Maybe you should get another B with the bench seat, then you'd have all the bases covered! :)