There was an exchange on the Laverdamania forum about the S and first type SF tank. I cannot access that forum any more since I was recently banned for not marching in step.I'm assuming he's talking about one of these - the first pic, black. The second S and third SF tanks fit just fine onto the S frame 26xx underneath them all.
Agreed that S and GT tanks not easily interchangeable as the GT frame is wider by about one inch (=25.4mm Paul!) at the rear of the tank.
Good question. Maybe they weren't sold for road use?"If fitted to a vehicle first used on or after 1st July 1973: - Made only of metal." so were 1974 750SFCs sold in the UK with fibreglass tanks?