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
We have no information that the electing consuls were restricted from admitting more than a definite number of plebeians to the senate; nor was there need for such a regulation, because the consuls themselves belonged to the nobility. On the other hand probably from the outset the consul was in virtue of his very position practically far less free, and far more bound by the opinions of his order and by custom, in the appointment of senators than the king.
The rule in particular, that the holding of the consulship should necessarily be followed by admission to the senate for life, if, as was probably the case at this time, the consul was not yet a member of it at the time of his election, must have in all probability very early acquired consuetudinary force. In like manner it seems to have become early the custom not to fill up the senators' places immediately on their falling vacant, but to revise and complete the roll of the senate on occasion of the census, consequently, as a rule, every fourth year; which also involved a not unimportant restriction on the authority entrusted with the selection.
The whole number of the senators remained as before, and in this the -conscripti- were also included; from which fact we are probably entitled to infer the numerical falling off of the patriciate.(14)
14. That the first consuls admitted to the senate 164 plebeians, is hardly to be regarded as a historical fact, but rather as a proof that the later Roman archaeologists were unable to point out more than 136 -gentes- of the Roman nobility (Rom, Forsch. i. 121).
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=26