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
Separation of the Orders in the Theatre
Of a kindred character was the formal separation of the places assigned to the senatorial order from those occupied by the rest of the multitude as spectators at the national festivals. It was the great Scipio, who effected this change in his second consulship in 560.
The national festival was as much an assembly of the people as were the centuries convoked for voting; and the circumstance that the former had no resolutions to pass made the official announcement of a distinction between the ruling order and the body of subjects--which the separation implied--all the more significant.
The innovation accordingly met with much censure even from the ruling class, because it was simply invidious and not useful, and because it gave a very manifest contradiction to the efforts of the more prudent portion of the aristocracy to conceal their exclusive government under the forms of civil equality.
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Reference address : https://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=10