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
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-.