The montefortino helmet was introduced by the Celts into Italy in the 4th C BC


"It seems the first scutum was adopted from the Samnites as Sallust wrote: "our ancestors... took their offensive and defensive weapons from the Samnites" and the author of Ineditum Vaticanum wrote: "We did not have the traditional Samnite scutum nor did we have the pila. But we fought with round shields and spears... But when we became involved in war with the Samnites we were equipped with the scutum and the pilum... so with foreign weapons and copied tactics we enslaved those who had developed a conceited pride in themselves." - Kraxis

Yes that may be the Roman’s story but one needs to consider the bigger picture. Sallust is referring to the Roman adoption of the Scutum post 1st Samnite War (341 BC). The Roman’s obviously gained a lot of exposure to the aforementioned during that war.
Interesting enough, archaeology testifies that the Samnites themselves had only adopted the long shield around 50 years previous. There was also the appearance of other strange / non-native military equipment like the triple disk breastplate that became such a famous Samnite icon. The tri disk being of religious significance to the Celts (see early examples of the Montefortino helmet)


Join the dots and fill in the gaps....it’s a hypothesis but isn’t it possible that the Samnites had adopted the long shield from the Celts who had carried that sytle of shield from the 6th century BC and whom the Samnites had been warring with to their north for over half a century previous. Shields similar to the Gallic shield called the ‘thureos’ in Greek (Pausanias, 10.22.6) have been found within their territory dating to around the beginning of the 4th c BC and the Celtic incursions. The Celtic Insubres were the first to enter Italy and they carried the thin ‘thureos’. Half a century later the Boii and other tribes entered Italy and carried the long oval shields ..and funnily enough, the Samnites in that part of the historical record were soon found with the same.

The proud conceited Roman’s were either ignorant that their arms had possibly come from the Celts or they deliberately chose to attribute them to the ‘more civilised’ ‘Italian cousins' than the ‘barbarian’ horde of Celts.
Considering that Sallust was writing during a period when Rome was at war with the Celts of Gaul, decendants of the sackers of their beloved city; and reading other Roman accounts of the Celts amounting to propaganda, the weight of evidence seems to support the hypothesis and the later 'Roman reasoning' imo.