My dads S-Type was ordered new in 1966 from the dealer to come fitted with a Webasto roof, and my understanding is that it was NOT fitted at the factory but by the dealer (who would have got a Webasto contractor in, or more likely sent off site to be done)
I do not know for export cars, which could well be different.
Ours had a little plate that said were it was installed attached to the front cant rail.
They still make them today, but I believe they only make them in one size, which fits the MGB.
There is a installation manual here just for interest.
http://www.tudorwebastosunroofs.com/dow ... _part1.pdf
http://www.tudorwebastosunroofs.com/dow ... _part2.pdf