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
The Servian Constitution
The first step, however, towards the amalgamation of the two portions of the people scarcely took place in the revolutionary way which their antagonism appeared to foreshadow. The reform of the constitution, which bears the name of king Servius Tullius, is indeed, as to its historical origin, involved in the same darkness with all the events of a period respecting which we learn whatever we know not by means of historical tradition, but solely by means of inference from the institutions of later times. But its character testifies that it cannot have been a change demanded by the plebeians, for the new constitution assigned to them duties alone, and not rights.
It must rather have owed its origin either to the wisdom of one of the Roman kings, or to the urgency of the burgesses that they should be delivered from exclusive liability to burdens, and that the non-burgesses should be made to share on the one hand in taxation--that is, in the obligation to make advances to the state (the -tributum-)--and rendering task-work, and on the other hand in the levy. Both were comprehended in the Servian constitution, but they hardly took place at the same time. The bringing in of the non-burgesses presumably arose out of the economic burdens; these were early extended to such as were "possessed of means" (-locupletes-) or "settled people" (-adsidui-, freeholders), and only those wholly without means, the "children-producers" (-proletarii-, -capite censi-) remained free from them.
Thereupon followed the politically more important step of bringing in the non-burgesses to military duty. This was thenceforth laid not upon the burgesses as such, but upon the possessors of land, the -tribules-, whether they might be burgesses or mere μέτοικοι; service in the army was changed from a personal burden into a burden on property. The details of the arrangement were as follow.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=12