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
Although this plebeian nobility, in the strict sense of the term, could only be formed after the curule offices were opened to plebeians, yet it exhibited in a short time, if not at the very first, a certain compactness of organization--doubtless because such a nobility had long been prefigured in the old senatorial plebeian families. The result of the Licinian laws in reality therefore amounted nearly to what we should now call the creation of a batch of peers.
Now that the plebeian families ennobled by their curule ancestors were united into one body with the patrician families and acquired a distinctive position and distinguished power in the commonwealth, the Romans had again arrived at the point whence they had started; there was once more not merely a governing aristocracy and a hereditary nobility--both of which in fact had never disappeared--but there was a governing hereditary nobility, and the feud between the gentes in possession of the government and the commons rising in revolt against the gentes could not but begin afresh.
And matters very soon reached that stage. The nobility was not content with its honorary privileges which were matters of comparative indifference, but strove after separate and sole political power, and sought to convert the most important institutions of the state--the senate and the equestrian order--from organs of the commonwealth into organs of the plebeio-patrician aristocracy.
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Reference address : https://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=6