I sent a "contact us" to Coker asking about these tires and they replied that they don't make a radial tire for a 21" Model A wheel. I replied asking why is it then, that you show one mounted on a 21" Model A wheel on your website? There has been a deafening silence since. So much for customer service! So much for honesty in advertising!
Late addition: Coker have responded and say that the only 21" tire they make is 5.00/600R21 and is too wide for the Model A wheel. That sounds more than reasonable so it looks like I might have to find a set of 19" wheels (not so easy here).