OK, I'll try to find that discussion in Laverdamania.
I sat the two "camelback" tanks side by side, one with original offset twist cap "S" and one with flip cap "SF", the only differences I can see are the caps and the lack of the air bleed pipe on the later tank, no mount differences or significant dimension changes. Photos here show them on the "wrong" frames, the SF tank on the S 26xx frame and the S tank on the 15xxx SF frame - no problems, the frames' differences don't seem to affect the fit of the tanks though clearly this does not consider clearance to the motor, carbs and other stuff under the tank or fit with the seat and panels.
Back to the original requester's question - I think the steel tanks from S, SF, SF2 (not GT) are all viable alternatives to his American Eagle fiberglass tank on an S frame to use till he decides he's going to get one made or some other approach.
I promise to stop beating this dead horse now.