I bet that's a regular T5. There are F8T4 lamps, but they're not 12". The diameter can vary slightly by manufacturer. For instance, I have a Sylvania UK that is too thick for the cover of my battery operated lantern, while a Sylvania US fits just fine. Your GE was probably made in the 1970s.
And I thought I had something really unusual here. I thought the etch looked like it originally said F8T5 but it was thinner than any of the other T5s I have. Anyway, thanks for letting me know!