I think, as I have four of them on Puff, presumably fitted when he was reconditioned around 1960.
Apparently it was a standard option on that generation of Fords if you were going to have singles all round instead of dual rears - when the more common 7.50 x 20 was standard. The size was also used on a lot of trailers and caravan stuff of the period.
They are difficult to get, but not impossible (I'm told) by a chap who got a new pair for his photographic trailer....
Anyone else here have some? I suppose I should grab four if anyone has a pile, although I've only seen US-manufactured ones.