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 proximate cause was of a military nature, as the various wars required a greater number of generals in chief command than the consular constitution allowed; but the change came to be of essential importance for the conflicts of the orders, and it may be that that military object was rather the pretext than the reason for this arrangement. According to the ancient law every burgess or μέτοικος liable to service might attain the post of an officer,(2) and in virtue of that principle the supreme magistracy, after having been temporarily opened up to the plebeians in the decemvirate, was now after a more comprehensive fashion rendered equally accessible to all freeborn burgesses.
2. Cf. I. VI. Political Effects of the Servian Military Organization
The question naturally occurs, what interest the aristocracy could have--now that it was under the necessity of abandoning its exclusive possession of the supreme magistracy and of yielding in the matter--in refusing to the plebeians the title, and conceding to them the consulate under this singular form?(3)
3. The defence, that the aristocracy clung to the exclusion of the plebeians from religious prejudice, mistakes the fundamental character of the Roman religion, and imports into antiquity the modern distinction between church and state.
The admittance of a non-burgess to a religious ceremony of the citizens could not indeed but appear sinful to the orthodox Roman; but even the most rigid orthodoxy never doubted that admittance to civic communion, which absolutely and solely depended on the state, involved also full religious equality. All such scruples of conscience, the honesty of which in themselves we do not mean to doubt, were precluded, when once they granted to the plebeians -en masse- at the right time the patriciate. This only may perhaps be alleged by way of excuse for the nobility, that after it had neglected the right moment for this purpose at the abolition of the monarchy, it was no longer in a position subsequently of itself to retrieve the neglect (Cf. II. I. The New Community).
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Reference address : https://www.ellopos.net/elpenor/rome/2-03-equalization-orders-aristocracy.asp?pg=4