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
Revolution in Political Processes
The year 664 had begun with a most abrupt rejection of the compromise offered by the insurgents and with the opening of a war of prosecutions, in which the most passionate defenders of patriotic selfishness, the capitalists, took vengeance on all those who were suspected of having counselled moderation and seasonable concession.
On the other hand the tribune Marcus Plautius Silvanus, who entered on his office on the 10th of December of the same year, carried a law which took the commission of high treason out of the hands of the capitalist jurymen, and entrusted it to other jurymen who were nominated by the free choice of the tribes without class-- qualification; the effect of which was, that this commission was converted from a scourge of the moderate party into a scourge of the ultras, and sent into exile among others its own author, Quintus Varius, who was blamed by the public voice for the worst democratic outrages--the poisoning of Quintus Metellus and the murder of Drusus.
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Reference address : https://www.ellopos.net/elpenor/rome/4-07-sulpician-revolution.asp?pg=36