Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
But a reaction set in: the sounds which had coalesced in Oscan, -d and -r, and the sounds which had coalesced in Latin, -g and -k, were again separated, and each was provided with its proper sign; -o and -u, for which from the first the Oscan alphabet had lacked separate signs, and which had been in Latin originally separate but threatened to coalesce, again became distinct, and in Oscan even the -i was resolved into two signs different in sound and in writing; lastly, the writing again came to follow more closely the pronunciation--the -s for instance among the Romans being in many cases replaced by -r.
Chronological indications point to the fifth century as the period of this reaction; the Latin -g for instance was not yet in existence about 300 but was so probably about 500; the first of the Papirian clan, who called himself Papirius instead of Papisius, was the consul of 418; the introduction of that -r instead of -s is attributed to Appius Claudius, censor in 442.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=29