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
Then admission to the patriciate was not in law foreclosed; now the highest object of plebeian ambition was to be admitted into the dumb appendage of the senate. The nature of the case implied that the governing aristocratic order, so far as it admitted plebeians at all, would grant the right of occupying seats in the senate not absolutely to the best men, but chiefly to the heads of the wealthy and notable plebeian families; and the families thus admitted jealously guarded the possession of the senatorial stalls.
While a complete legal equality therefore had subsisted within the old burgess-body, the new burgess-body or former --metoeci-- came to be in this way divided from the first into a number of privileged families and a multitude kept in a position of inferiority. But the power of the community now according to the centuriate organization came into the hands of that class which since the Servian reform of the army and of taxation had borne mainly the burdens of the state, namely the freeholders, and indeed not so much into the hands of the great proprietors or into those of the small cottagers, as into those of the intermediate class of farmers--an arrangement in which the seniors were still so far privileged that, although less numerous, they had as many voting- divisions as the juniors.
While in this way the axe was laid to the root of the old burgess-body and their clan-nobility, and the basis of a new burgess-body was laid, the preponderance in the latter rested on the possession of land and on age, and the first beginnings were already visible of a new aristocracy based primarily on the actual consideration in which the families were held--the future nobility. There could be no clearer indication of the fundamentally conservative character of the Roman commonwealth than the fact, that the revolution which gave birth to the republic laid down at the same time the primary outlines of a new organization of the state, which was in like manner conservative and in like manner aristocratic.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=38