Howdy. I got an STF lemon from Four Winds a couple of years ago. An employee I spoke with at the time said that it was on C-35, which was what they were using on all their trees except for a few mandarins. As of then they were no longer using the Cuban shaddock which had been their primary rootstock for decades. I was told that they had never used Flying Dragon. Four Winds had never been particularly forthcoming with rootstock information in my experience. A super secret trade secret apparently. It took at least half a dozen calls to find someone who was willing to talk about their rootstock(s).
I'm keeping the STF lemon in a pot. I think it'd be too vigorous to join the rest of my citrus trees, all planted in ground on Flying Dragon.
Steve H. USDA zone 3, SW WI USA