The FT diesel was designed and the FT-103 demonstrator and early production models built with drawbars betwen the A and B units. In fact, the early ones didn't even have doors between the A and B units!! After production began, Santa Fe began looking into a way to add couplers in place of the drawbars, and at some point GM worked out a way to do it and added it as an option to the FT. (I think Santa Fe modified some on their own first?)
GM F units were the FT, F-2, F-3, F-7 and F-9...however during the transition from the F-3 to F-7 some engines were produced that were sometimes referred to by railroads (GN for example) and railfans as "F-5s" but this IIRC was never an official GM designation. So it may be that GM used "FS" as some internal name for FT units with couplers at both ends, but everything I've ever seen just refers to them as "FT"s.