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
But it is significant that in this case, where the sudden amalgamation of a number of isolated cantons into a new political unity might have so naturally suggested the idea of a representative constitution in the modern sense, no trace of any such idea occurs; in fact the very opposite course was followed,(12) and the communal organization was simply reproduced in a far more absurd manner than before.
12. Even from our scanty information, the best part of which is given by Diodorus, p. 538 and Strabo, v. 4, 2, this is very distinctly apparent; for example, the latter expressly says that the burgess-body chose the magistrates. That the senate of Italia was meant to be formed in another manner and to have different powers from that of Rome, has been asserted, but has not been proved. Of course in its first composition care would be taken to have a representation in some degree uniform of the insurgent cities; but that the senators were to be regularly deputed by the communities, is nowhere stated. As little does the commission given to the senate to draw up a constitution exclude its promulgation by the magistrates and ratification by the assembly of the people.
Nowhere perhaps is it so clearly apparent as in this instance, that in the view of antiquity a free constitution was inseparable from the appearance of the sovereign people in person in the primary assemblies, or from a city; and that the great fundamental idea of the modern republican-constitutional state, viz. the expression of the sovereignty of the people by a representative assembly--an idea without which a free state would be a chaos--is wholly modern. Even the Italian polity, although in its somewhat representative senates and in the diminished importance of the comitia it approximated to a free state, never was able in the case either of Rome or of Italia to cross the boundary-line.
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Reference address : https://www.ellopos.net/elpenor/rome/4-07-sulpician-revolution.asp?pg=24