what is the difference between a 55 and a 55g idle jet ?

The size (bore) is indicated by the number, so they will "flow" the same amount (if not worn etc).
But are the THREADS the same (O.D.)? Are they exactly the same Length & Shape?

30 PICT- (and all earlier Beetle Solexes I think) used "55" (or whatever bore); but 34 PICT- onwards used the "g55" versions. The 30 PICT- had the cut-off valve directly on top of the idle jet, presumably cutting off fuel flow at the jet, so perhaps the design of the top of the jet was different...? (Have not owned a car with a 30 PICT-)

Some diagrams in old VW/Solex/Pierburg manuals label the various parts with letters, and "g" always seemes used for the pilot/idle jet, so perhaps it just became a shorthand indication for it.