We had our GTL Zenith 28 IF (V05 078B) serviced recently, but I can't remember if it had one or two air jets.
In the Haynes manual, I can see only one 'air jet' in its exploded diagram for the early engines (680 747cc and 800 845cc). Unfortunately it doesn't say which carb variants were used on which of these engines (just the type and model).
According to the Haynes supplement, the V05 075B was fitted to the 800 845cc engine (as were the Solex 32 and Zenith 32), while the 688-C1E 1108cc engine had various Zenith 28 IF carbs:
V05 073A(C) .... F6
V05 078(A)(P) .. GTL
V05 082 .......... F6 and GTL
V05 83 ............ F6
V05 902 .......... F6 and GTL
V05 080 .......... GPL
Apart from the V05 902 (which isn't mentioned on la4ldesylvie.fr), this agrees with that website, but tells us whether they were used on the F6 or the GTL.