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
Centuries and Curies
The community thus acquired by the change of constitution rights of the greatest importance: the right of annually designating its presidents, and that of deciding in the last instance regarding the life or death of the burgess. But the body which acquired these rights could not possibly be the community as it had been hitherto constituted--the patriciate which had practically become an order of nobility.
The strength of the nation lay in the "multitude" (-plebs-) which already comprehended in large numbers people of note and of wealth. The exclusion of this multitude from the public assembly, although it bore part of the public burdens, might be tolerated as long as that public assembly itself had no very material share in the working of the state machine, and as long as the royal power by the very fact of its high and free position remained almost equally formidable to the burgesses and to the μέτοικοι and thereby maintained equality of legal redress in the nation.
But when the community itself was called regularly to elect and to decide, and the president was practically reduced from its master to its commissioner for a set term, this relation could no longer be maintained as it stood; least of all when the state had to be remodelled on the morrow of a revolution, which could only have been carried out by the co-operation of the patricians and the μέτοικοι.
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Reference address : http://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=19