On pg. 29, fn. 22 I mention that according to the testimony of Velius Longus the letter Z was not aliena (Latino) sermoni and was used in the Carmen Saliare. Subsequently it was banished, perhaps by Appius Claudius Caecus, yielding its place in the alphabet to G, only to be reintroduced at the end of the alphabet in the 1st cent. BCE. In addition, I should have noted that there is one probable epigraphic example of original Z from the Very Old Latin period. A bowl found on the Esquiline in the area of the Villa Altieri in 1876 and dated not later than the VIIth century BCE bears the inscription ZKA. If this text is Latin—the null hypothesis for a inscription of this date and time— this would be, to my knowledge, the sole epigraphical example of Z in its first run in the Latin alphabet. Colonna 1980 suggests that ZKA stands for SKA with Z for S as in Faliscan.
Colonna, Giovanni, 1980. L'aspetto epigrafico del Lapis Satricanus, in Lapis Satricanus (Archeologische Studien van he Nederlands Institut te Rome, Scripta Minora V) 's Gravenhage 41–69.