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 same character of fanatical exasperation marks other events of this epoch, the historical significance of which can no longer be apprehended in the lying family narratives; such as the predominance of the Fabian clan which furnished one of the two consuls from 269 to 275, and the reaction against it, the emigration of the Fabii from Rome, and their annihilation by the Etruscans on the Cremera (277). Still more odious was the murder of the tribune of the people, Gnaeus Genucius, who had ventured to call two consulars to account, and who on the morning of the day fixed for the impeachment was found dead in bed (281).
The immediate effect of this misdeed was the Publilian law (283), one of the most momentous in its consequences with which Roman history has to deal. Two of the most important arrangements--the introduction of the plebeian assembly of tribes, and the placing of the -plebiscitum- on a level, although conditionally, with the formal law sanctioned by the whole community--are to be referred, the former certainly, the latter probably, to the proposal of Volero Publilius the tribune of the people in 283.
The plebs had hitherto adopted its resolutions by curies; accordingly in these its separate assemblies, on the one hand, the voting had been by mere number without distinction of wealth or of freehold property, and, on the other hand, in consequence of that standing side by side on the part of the clansmen, which was implied in the very nature of the curial assembly, the clients of the great patrician families had voted with one another in the assembly of the plebeians.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=25