When I fitted 15" Punto HGT alloys to my B a few years ago, I had to use the bolts that come from the Punto. I did have the original B alloys and the bolts looked almost the same.
Hmmm, I have just paid £30 for a supposedly good set of locking wheel nuts for my new alloys (could have paid £18 fo a so-so set), got them home and went to fit them to find that three of them were fine but one of them didnt match the locking key Now waiting for a complete new set.
When I fitted 15" Punto HGT alloys to my B a few years ago, I had to use the bolts that come from the Punto. I did have the original B alloys and the bolts looked almost the same.
The new Grand Punto has a PCD 4x100, so you will need special bolts if you want to fit them to a Barchetta 4x98. You could also have issues with the center hub as I did with my 17" Grand Punto Sporting alloys.