A rare example to be sure! Even most 64 Falcons, for instance, had a generator. Police versions would have been among the first cars. Top of the line models like the Galaxie or big Mercury's in '63 perhaps, but not common. F-100 trucks, not likely. Pretty cool the Cobra ranks right up there with the best! But plug in style??? Hmmm, searching for something to confirm that (Google to the rescue)!
I sure don't recall a plug in on the early Fords I worked on... The first Mustangs were generators.