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
One consequence of this amalgamation of the patricians and plebeians in a new corporation of Roman burgesses was the conversion of the old burgesses into a clan-nobility, which was incapable of receiving additions or even of filling up its own ranks, since the nobles no longer possessed the right of passing decrees in common assembly and the adoption of new families into the nobility by decree of the community appeared still less admissible.
Under the kings the ranks of the Roman nobility had not been thus closed, and the admission of new clans was no very rare occurrence: now this genuine characteristic of patricianism made its appearance as the sure herald of the speedy loss of its political privileges and of its exclusive estimation in the community.
The exclusion of the plebeians from all public magistracies and public priesthoods--while they were admissible to the position of officers and senators--and the maintenance, with perverse obstinacy, of the legal impossibility of marriage between old burgesses and plebeians, further impressed on the patriciate from the outset the stamp of an exclusive and wrongly privileged aristocracy.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=29