So I naturally assumed that there was a bust known with certainty to be that of Zeno of Citium, but the fact is that apart from the inscription Ζήνων there is no positive identification on any of them to suggest which of the philosophers who held that name were sculpted. It could be Zeno of Citium, Zeno of Sidon, or Zeno of Elea (student of Parmenides and author of Zeno's Paradox).
Piso died in Italy ~30 years after Zeno of Sidon died in Athens. There are 122 years between Piso and Vesuvius--who lived there in the meantime? Why did they preserve Philodemus' library? Were they Epicurean? If so, why is there a bust of the founder of Stoicism in the Villa? Perhaps Piso or his successors thought it was Zeno of Sidon.
I'll need to review Frischer's book. This kind of blows my mind--Philodemus studied under Zeno of Sidon, a bust of someone named Zeno is in the Villa that houses his library, and yet the whole world over knows it as a bust of Zeno of Citium...because it doesnt look Epicurean enough! If Bernard Frischer's is the last word on this subject, I don't think this investigation is over yet.