Hi Melvin,
The FT was the first Electro-Motive freight diesel, demonstrated in 1939 and entering production in 1940. It was also the model that introduced what became known as the “bulldog nose” on cab diesels. The bodies of the F2 through the F9 maintained the same basic shape as the FT with only minor changes in dimensions, particularly truck centers. Collectively, these are all known as “F units.” There were many variations in external details over the course of F- unit production but only the FTs had the four-porthole side panels and the flush-mounted radiator fans in the roof. The book I referred to before has a good guide to the different types of F units.
So long,
Andy