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
To facilitate the levying of the infantry, the city was distributed into four "parts" (-tribus-); by which the old triple division was superseded, at least so far as concerned its local significance. These were the Palatine, which comprehended the height of that name along with the Velia; the Suburan, to which the street so named, the Carinae, and the Caelian belonged; the Esquiline; and the Colline, formed by the Quirinal and Viminal, the "hills" as contrasted with the "mounts" of the Capitol and Palatine. We have already spoken of the formation of these regions(8) and shown how they originated out of the ancient double city of the Palatine and the Quirinal.
8. Cf. I. IV. Oldest Settlements In the Palatine and Suburan Regions
By what process it came to pass that every freeholder burgess belonged to one of those city-districts, we cannot tell; but this was now the case; and that the four regions were nearly on an equality in point of numbers, is evident from their being equally drawn upon in the levy. This division, which had primary reference to the soil alone and applied only inferentially to those who possessed it, was merely for administrative purposes, and in particular never had any religious significance attached to it; for the fact that in each of the city-districts there were six chapels of the enigmatical Argei no more confers upon them the character of ritual districts than the erection of an altar to the Lares in each street implies such a character in the streets.
Each of these four levy-districts had to furnish approximately the fourth part not only of the force as a whole, but of each of its military subdivisions, so that each legion and each century numbered an equal proportion of conscripts from each region, in order to merge all distinctions of a gentile and local nature in the one common levy of the community and, especially through the powerful levelling influence of the military spirit, to blend the μέτοικοι and the burgesses into one people.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=15