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Probably because the F2 had pd carbs. Will the F1 tap accept the securing nut so you could fit the F1 tap into the gland nut from the F2 tap.
The vac on this one is pretty small and looks well within the outlet stub in your original picture, so from that aspect looks possible.I'll keep an eye on here to see if you solve it before I get to look at this one. I'm working on it next week so if more spec's are needed then I'll post them for you. Nigel.