a few days late, but I want to add:
as far as I found out, a Solex 28 PBJ or even PBI never was produced.
I think, that should be false information, even in the "ABARTH 595 695" book of Deganello/Donati
For some time I thought it could be a Solex 28 PIB from Fiat 600, but that type does not fit on the Fiat 500.
But what is fitting very well is the Solex C 28 IB-2 and I am happy I found one some years ago
That is the carb that had been mounted on the "small" Abarth 595...
The ABARTH 595 esse-esse had the Solex F34-PBIC with its very special jetting
some pics:
1st pic: comparison Solex C28IB-2 - Weber 28 IMB
2nd pic: Solex C28IB-2 on my 650 engine
3rd pic: Solex F34PBIC on my 595 engine