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
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.