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
In regard however to this imposing republican equality we must not
overlook the fact that it was to a considerable extent only formal,
and that an aristocracy of a very decided stamp grew out of it or
rather was contained in it from the very first. The non-patrician
families of wealth and consideration had long ago separated from the
plebs, and leagued themselves with the patriciate in the participation
of senatorial rights and in the prosecution of a policy distinct from
that of the plebs and very often counteracting it.
The Licinian laws
abrogated the legal distinctions within the ranks of the aristocracy,
and changed the character of the barrier which excluded the plebeian
from the government, so that it was no longer a hindrance unalterable
in law, but one, not indeed insurmountable, but yet difficult to be
surmounted in practice. In both ways fresh blood was mingled with
the ruling order in Rome; but in itself the government still remained,
as before, aristocratic.