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
The Plebeian Opposition
It is therefore quite true that the immediate effect of the revolution was to establish the aristocratic government. It is not, however, the whole truth. While the majority of contemporaries probably thought that the revolution had brought upon the plebeians only a more rigid despotism, we who come afterwards discern in that very revolution the germs of young liberty. What the patricians gained was gained at the expense not of the community, but of the magistrate's power.
It is true that the community gained only a few narrowly restricted rights, which were far less practical and palpable than the acquisitions of the nobility, and which not one in a thousand probably had the wisdom to value; but they formed a pledge and earnest of the future. Hitherto the μέτοικοι had been politically nothing, the old burgesses had been everything; now that the former were embraced in the community, the old burgesses were overcome; for, however much might still be wanting to full civil equality, it is the first breach, not the occupation of the last post, that decides the fall of the fortress. With justice therefore the Roman community dated its political existence from the beginning of the consulate.
While however the republican revolution may, notwithstanding the aristocratic rule which in the first instance it established, be justly called a victory of the former μέτοικοι or the -plebs-, the revolution even in this respect bore by no means the character which we are accustomed in the present day to designate as democratic. Pure personal merit without the support of birth and wealth could perhaps gain influence and consideration more easily under the regal government than under that of the patriciate.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=37