Hello,
I did a little research in
ePER and it seems it's the part marked 2:
View attachment 461567
It has a typical thread underneath:
View attachment 461568
You will need it also for the tool holder you put in the wheel. However no idea if it's exactly the same, as there are 2 different part numbers.
It looks to me the thread got some rust and was stuck and the plastic holder just spins around the bolt hexagonal thread. I assume
The 499 option on the list is the tyre repair kit, so I think you'll need this one: 46761191 and not the 51755749.
Double check before you purchase!
FYI,
ePER is the FIAT spare parts catalogue. The link is on the bottom of the forum banner. Search window is top left of the screen. You can browse by models and categories or, which I recommend, search with the VIN number to have the parts that fit your car. In every moment you can push the 'Show all results' button below the parts list to have all of them in this category. On top of my screenshot you can find through which categories you have to go to get to this diagram / parts list.