I have seen some of these on my Fiats. If the stud is threaded with a coarse thredm and painted its not for a nut, but is a carpet fixing you have lost. Thing like a mushroom, about 20mm across. The stem is a hollow tube and these push over the 'thread; and hold the carpet. Not the best design ever and they fall off without much provocation. There are similar fastenings used in the wheel arches to hold the arch liners. Some of the fixings in the wheel arch resemble plastic nuts but these too push on in most cases.