I've a foggy memory of the VSCC permitting radials when fitted originally, but the X was the first and post war so that can't be right. They used to permit radials on Austin 7s, they might still do so for the racing cars but I'm very much out of touch. There's a chance they'd still give you an entry in rallies on radials, can but ask, as long as you promised not to win.
https://en.wikipedia.org/wiki/Radial_tireMy other foggy memory is that the 2CV was a 400 when it first came out.
Interesting to see the Stratos as the first with a "wide radial tyre". Can anyone tell the full story of that one?