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
In a similar way soon afterwards (about 625) the senators were by decree of the people enjoined on admission to the senate to surrender their public horse, and thereby to renounce their privileged place in the voting of the eighteen equestrian centuries.(2)
2. Cf. III. XI. The Nobility in Possession of the Equestrian Centuries
These measures, directed to the emancipation of the electors from the ruling aristocratic order, may perhaps have seemed to the party which suggested them the first step towards a regeneration of the state; in fact they made not the slightest change in the nullity and want of freedom of the legally supreme organ of the Roman community; that nullity indeed was only the more palpably evinced to all whom it did or did not concern.
Equally ostentatious and equally empty was the formal recognition accorded to the independence and sovereignty of the burgesses by the transference of their place of assembly from the old Comitium below the senate-house to the Forum (about 609). But this hostility between the formal sovereignty of the people and the practically subsisting constitution was in great part a semblance.
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Reference address : https://www.ellopos.net/elpenor/rome/4-02-reform-tiberius-gracchus.asp?pg=7