K3. F's came out in 1975, and the F3 was 1978. But they may have stamped the airbox 750F (again, unless I'm having a spasmodic brain fart) to denote it as being a four. I am pretty sure that's why the 350F was designated "F", to separate it from the 350 (twin), as there wasn't much supersport about it (at least not compared with the 550F and 750F, 4-1 exhaust, etc.). Just speculating here.