The use of <σδ> in the Oscan inscription from Messina I have already mentioned (see my post Voiced s in the Oenotrian inscription from Tortora. Part one) stands out in stark contrast to the ways of representing voiced s in Oscan writing systems, that is s in the native alphabet, e.g. eíseís, ζ in the Greek, e.g. ειζιδομ, and z in the Latin, e.g. egmazum in the Tabula Bantina (for further details see Carl Darling Buck, A Grammar of Oscan and Umbrian. With a Collection of Inscriptions and a Glossary, 1904; Vittore Pisani, Manuale storico della lingua latina. IV. Le lingue dell’Italia antica oltre il latino, 2nd edition, 1964; Michel Lejeune, Inscriptions de Rossano di Vaglio 1971, in Accademia Nazionale dei Lincei, Anno CCCLXIII, 1971, Serie ottava, Rendiconti, Classe di Scienze morali, storiche e filologiche, Vol. XXVI, 1972).
In view of this epigraphic feature shared between the Messina and Tortora inscriptions and considering that the use of <σδ> is the only example in the Oenotrian inscription from Tortora of deviation from the usual representation of s, we may conclude with certainty that both spellings constitute an attempt to represent the voiced allophone of s in two specific environments, that is intervocalically and word-finally.
In the Italic branch of the Indo-European family intervocalic s underwent voicing, stopping at this stage in Oscan, but rhotacising in Latin and Umbrian, while final s was retained in Oscan and Latin, but only partially in Umbrian, as is shown by the fact that it remained unchanged in the older Iguvine Tables (I-IV), but rhotacised in the later ones (V-VIII).
Italic voicing of final s is documented by final <z> in two archaic Faliscan anthroponyms (qunoz iatinoz) inscribed on a chalice coming from the necropolis of Magliano Sabina (for further details see Paolo Poccetti, Il vaso iscritto dalla necropoli di Magliano Sabina. Contributo ai rapporti tra l’ambiente falisco e quello sabino arcaico, in Una nuova iscrizione da Magliano Sabina. Scrittura e cultura nella valle del Tevere, P. Santoro ed., 2008) but it can also be inferred from the Umbrian words showing final r (e.g. prinuatur), because the development of rhotacism necessarily implies a previous stage with voiced s.