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
Enlargement of the City of Rome - Servian Wall
While the Latin stock was thus tending towards union under the leadership of Rome and was at the same time extending its territory on the east and south, Rome itself, by the favour of fortune and the energy of its citizens, had been converted from a stirring commercial and rural town into the powerful capital of a flourishing country.
The remodelling of the Roman military system and the political reform of which it contained the germ, known to us by the name of the Servian constitution, stand in intimate connection with this internal change in the character of the Roman community. But externally also the character of the city cannot but have changed with the influx of ampler resources, with the rising requirements of its position, and with the extension of its political horizon.
The amalgamation of the adjoining community on the Quirinal with that on the Palatine must have been already accomplished when the Servian reform, as it is called, took place; and after this reform had united and consolidated the military strength of the community, the burgesses could no longer rest content with entrenching the several hills, as one after another they were filled with buildings, and with possibly also keeping the island in the Tiber and the height on the opposite bank occupied so that they might command the course of the river.
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Reference address : https://www.ellopos.net/elpenor/rome/1-07-hegemony-rome-latium.asp?pg=14