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 New Community
This, in short, was the time when the Roman burgess-body in the later sense of the term originated. The plebeians had hitherto been μέτοικοι who were subjected to their share of taxes and burdens, but who were nevertheless in the eye of the law really nothing but tolerated aliens, between whose position and that of foreigners proper it may have seemed hardly necessary to draw a definite line of distinction.
They were now enrolled in the lists as burgesses liable to military service, and, although they were still far from being on a footing of legal equality--although the old burgesses still remained exclusively entitled to perform the acts of authority constitutionally pertaining to the council of elders, and exclusively eligible to the civil magistracies and priesthoods, nay even by preference entitled to participate in the usufructs of burgesses, such as the joint use of the public pasture--yet the first and most difficult step towards complete equalization was gained from the time when the plebeians no longer served merely in the common levy, but also voted in the common assembly and in the common council when its opinion was asked, and the head and back of the poorest μέτοικος were as well protected by the right of appeal as those of the noblest of the old burgesses.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=28