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 senate was not affected by the reform of the constitution in the same way as the community. The previously existing college of elders not only continued exclusively patrician, but retained also its essential prerogatives--the right of appointing the interrex, and of confirming or rejecting the resolutions adopted by the community as constitutional or unconstitutional.
In fact these prerogatives were enhanced by the reform of the constitution, because the appointment of the magistrates also, which fell to be made by election of the community, was thenceforth subject to the confirmation or rejection of the patrician senate. In cases of appeal alone its confirmation, so far as we know, was never deemed requisite, because in these the matter at stake was the pardon of the guilty and, when this was granted by the sovereign assembly of the people, any cancelling of such an act was wholly out of the question.
But, although by the abolition of the monarchy the constitutional rights of the patrician senate were increased rather than diminished, there yet took place--and that, according to tradition, immediately on the abolition of the monarchy--so far as regards other affairs which fell to be discussed in the senate and admitted of a freer treatment, an enlargement of that body, which brought into it plebeians also, and which in its consequences led to a complete remodelling of the whole. From the earliest times the senate had acted also, although not solely or especially, as a state-council; and, while probably even in the time of the kings it was not regarded as unconstitutional for non- senators in this case to take part in the assembly,(12) it was now arranged that for such discussions there should be associated with the patrician senate (-patres-) a number of non-patricians "added to the roll" (-conscripti-).
12. Cf. I. V. The Senate as State Council
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=23