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 cavalry was similarly dealt with. The number of divisions in it was tripled, and the only difference in this case was that the six divisions already existing with the old names (-Tities-, -Ramnes-, -Luceres- -primi- and -secundi-) were left to the patricians, while the twelve new divisions were formed chiefly from the non-burgesses. The reason for this difference is probably to be sought in the fact that at that period the infantry were formed anew for each campaign and discharged on their return home, whereas the cavalry with their horses were on military grounds kept together also in time of peace, and held their regular drills, which continued to subsist as festivals of the Roman equites down to the latest times.(7)
7. For the same reason, when the levy was enlarged after the admission of the Hill-Romans, the equites were doubled, while in the infantry force instead of the single "gathering" (-legio-) two legions were called out (vi. Amalgamation of the Palatine and Quirinal Cities).
Accordingly the squadrons once constituted were allowed, even under this reform, to keep their ancient names. In order to make the cavalry accessible to every burgess, the unmarried women and orphans under age, so far as they had possession of land, were bound instead of personal service to provide the horses for particular troopers (each trooper had two of them), and to furnish them with fodder. On the whole there was one horseman to nine foot-soldiers; but in actual service the horsemen were used more sparingly.
The non-freeholders (-adcensi-, people standing at the side of the list of those owing military service) had to supply the army with workmen and musicians as well as with a number of substitutes who marched with the army unarmed (-velati-), and, when vacancies occurred in the field, took their places in the ranks equipped with the weapons of the sick or of the fallen.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=14