Attic Helmet too good to be true?

[Tarbicus]

That's only true for Principate artwork. Even early Principate artwork still accurately depicts some known helmets that were in use at the time.

Do you mean monumental artwork or provincial smaller scale sculptures? In the early principate were the classic attics shown or more traditional hellenistic ones (a question not a statement)? To be honest, I can't recall even an accurate Montefortino, and those that people point to crying "Montefortino!" look dubious to me and more hellenistic. Now and again I'm sure I've also seen a minor detail dotted about that could be, but nothing conclusive.

I think it's safe to assume that the most familiar form of helmets that would've been known to civilians in Rome would be those of the Praetorian Guard. So, there's a chance that the sculptors merely assumed that all legionaries wore identical helmets.

Not necessarily. Robinson believes they may also or alternatively have been helmets for vigiles.