Complex sentence: The inscription in Etruscan letters reads "eluveitie", which has been interpreted as the Etruscan form of the Celtic "(h) elvetios" ("the Helvetian"), presumably referring to a man of Helvetian descent living in Mantua.
Split into simpler sentences: The inscription in Etruscan letters reads "eluveitie", which has been interpreted as the Etruscan form of the Celtic "(h) elvetios" (i.e. "the Helvetian"). One can assume that "the Helvetian" was a man of Helvetian descent living in Mantua.