I suggest you take a look at Nails for the dead: a polysemic account of an ancient funerary practice by Silvia Alfayé. The paper focuses on the use of nails in Roman funerary practices, but in section 3 she offers a nice overview on the ritual/magical uses of nails in general, including the inscribed bronze nails you are asking about, which she describes as:
These are the so-called chiodi magici, which are
10–20cm long, made of bronze or iron, decorated with geometrical
patterns, sigla and/or charakteres, and show no signs of wear.
She then lists various possible uses which have been suggested for them: as tools of divination, as votive objects, as amulets. It is even suggested that some of them were used to cure epilepsy, using a technique cited by Pliny, or that they were just more sophisticated versions of simpler, non-inscribed funerary nails.