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 the case of the original συνοικισμός the annexed community was recognized after the union as at least a tribe (part) of the new burgess-body, and thus had in some sense a continued political existence; but this course was not followed in the case of the Hill-Romans or in any of the later processes of annexation. After the union the Roman community continued to be divided as formerly into three tribes, each containing ten wardships (-curiae-); and the Hill-Romans--whether they were or were not previously distributed into tribes of their own--must have been inserted into the existing tribes and wardships.
This insertion was probably so arranged that, while each tribe and wardship received its assigned proportion of the new burgesses, the new burgesses in these divisions were not amalgamated completely with the old; the tribes henceforth presented two ranks: the Tities, Ramnes, and Luceres being respectively subdivided into first and second (-priores-, -posteriores-). With this division was connected in all probability that arrangement of the organic institutions of the community in pairs, which meets us everywhere.
The three pairs of Sacred Virgins are expressly described as representatives of the three tribes with their first and second ranks; and it may be conjectured that the pair of Lares worshipped in each street had a similar origin. This arrangement is especially apparent in the army: after the union each half-tribe of the tripartite community furnished a hundred horsemen, and the Roman burgess cavalry was thus raised to six "hundreds," and the number of its captains probably from three to six.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=2