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
Election of Officers in the Comitia
Among the innovations originated by the government--which were none the less innovations, that almost uniformly they changed not the letter, but merely the practice of the existing constitution--the most prominent were the measures by which the filling up of officers' posts as well as of civil magistracies was made to depend not, as the letter of the constitution allowed and its spirit required, simply on merit and ability, but more and more on birth and seniority.
As regards the nomination of staff-officers this was done not in form, but all the more in substance. It had already, in the course of the previous period, been in great part transferred from the general to the burgesses;(13) in this period came the further step, that the whole staff-officers of the regular yearly levy--the twenty-four military tribunes of the four ordinary legions--were nominated in the -comitia tributa-.
13. Cf. II. III. Increasing Powers of the Burgesses
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Reference address : https://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=14