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
In these divisions, which were based throughout on the possession of land, the voters were exclusively freeholders: but they voted without distinction as to the size of their possession, and just as they dwelt together in villages and hamlets. Consequently, this assembly of the tribes, which otherwise was externally modelled on that of the curies, was in reality an assembly of the independent middle class, from which, on the one hand, the great majority of freedmen and clients were excluded as not being freeholders, and in which, on the other hand, the larger landholders had no such preponderance as in the centuries.
This "meeting of the multitude" (-concilium plebis-) was even less a general assembly of the burgesses than the plebeian assembly by curies had been, for it not only, like the latter, excluded all the patricians, but also the plebeians who had no land; but the multitude was powerful enough to carry the point that its decree should have equal legal validity with that adopted by the centuries, in the event of its having been previously approved by the whole senate.
That this last regulation had the force of established law before the issuing of the Twelve Tables, is certain; whether it was directly introduced on occasion of the Publilian -plebiscitum-, or whether it had already been called into existence by some other--now forgotten--statute, and was only applied to the Publilian -plebiscitum- cannot be any longer ascertained. In like manner it remains uncertain whether the number of tribunes was raised by this law from two to four, or whether that increase had taken place previously.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=27