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
There is no tradition of any corresponding increase to the infantry; but to this origin we may refer the subsequent custom of calling out the legions regularly two by two, and this doubling of the levy probably led to the rule of having not three, as was perhaps originally the case, but six leaders-of-division to command the legion. It is certain that no corresponding increase of seats in the senate took place: on the contrary, the primitive number of three hundred senators remained the normal number down to the seventh century; with which it is quite compatible that a number of the more prominent men of the newly annexed community may have been received into the senate of the Palatine city.
The same course was followed with the magistracies: a single king presided over the united community, and there was no change as to his principal deputies, particularly the warden of the city. It thus appears that the ritual institutions of the Hill-city were continued, and that the doubled burgess-body was required to furnish a military force of double the numerical strength; but in other respects the incorporation of the Quirinal city into the Palatine was really a subordination of the former to the latter.
If we have rightly assumed that the contrast between the Palatine old and the Quirinal new burgesses was identical with the contrast between the first and second Tities, Ramnes, and Luceres, it was thus the -gentes-of the Quirinal city that formed the "second" or the "lesser." The distinction, however, was certainly more an honorary than a legal precedence. At the taking of the vote in the senate the senators taken from the old clans were asked before those of the "lesser." In like manner the Colline region ranked as inferior even to the suburban (Esquiline) region of the Palatine city; the priest of the Quirinal Mars as inferior to the priest of the Palatine Mars; the Quirinal Salii and Luperci as inferior to those of the Palatine.
It thus appears that the συνοικισμός, by which the Palatine community incorporated that of the Quirinal, marked an intermediate stage between the earliest συνοικισμός by which the Tities, Ramnes, and Luceres became blended, and all those that took place afterwards. The annexed community was no longer allowed to form a separate tribe in the new whole, but it was permitted to furnish at least a distinct portion of each tribe; and its ritual institutions were not only allowed to subsist--as was afterwards done in other cases, after the capture of Alba for example--but were elevated into institutions of the united community, a course which was not pursued in any subsequent instance.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=3