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 extraordinary filling up of the senate was undertaken by the monarch alone. In the case of the ordinary additions he secured to himself a permanent influence through the circumstance, that the electoral colleges were bound by law(23) to give their votes to the first twenty candidates for the quaestorship who were provided with letters of recommendation from the monarch; besides, the crown was at liberty to confer the honorary rights attaching to the quaestorship or to any office superior to it, and consequently a seat in the senate in particular, by way of exception even on individuals not qualified.
23. This certainly had reference merely to the elections for the years 711 and 712 (Staatsrecht, ii. a 730); but the arrangement was doubtless meant to become permanent.
The selection of the extraordinary members who were added naturally fell in the main on adherents of the new order of things, and introduced, along with -equites- of respectable standing, various dubious and plebeian personages into the proud corporation--former senators who had been erased from the roll by the censor or in consequence of a judicial sentence, foreigners from Spain and Gaul who had to some extent to learn their Latin in the senate, men lately subaltern officers who had not previously received even the equestrian ring, sons of freedmen or of such as followed dishonourable trades, and other elements of a like kind.
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Reference address : http://www.ellopos.net/elpenor/rome/5-11-old-republic-new-monarchy.asp?pg=46