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 custom of fighting in three divisions was retained, but, while previously each division had formed a distinct corps, it was in future left to the general to distribute the cohorts, of which he had the disposal, in the three lines as he thought best. Military rank was determined solely by the numerical order of the soldiers and of the divisions. The four standards of the several parts of the legion--the wolf, the ox with a man's head, the horse, the boar--which had hitherto probably been carried before the cavalry and the three divisions of heavy infantry, disappeared; there came instead the ensigns of the new cohorts, and the new standard which Marius gave to the legion as a whole--the silver eagle.
While within the legion every trace of the previous civic and aristocratic classification thus disappeared, and the only distinctions henceforth occurring among the legionaries were purely military, accidental circumstances had some decades earlier given rise to a privileged division of the army alongside of the legions-- the bodyguard of the general. Hitherto selected men from the allied contingents had formed the personal escort of the general; the employment of Roman legionaries, or even men voluntarily offering themselves, for personal service with him was at variance with the stern disciplinary obligations of the mighty commonwealth.
But when the Numantine war had reared an army demoralized beyond parallel, and Scipio Aemilianus, who was called to check the wild disorder, had not been able to prevail on the government to call entirely new troops under arms, he was at least allowed to form, in addition to a number of men whom the dependent kings and free cities outside of the Roman bounds placed at his disposal, a personal escort of 500 men composed of volunteer Roman burgesses (p. 230). This cohort drawn partly from the better classes, partly from the humbler personal clients of the general, and hence called sometimes that of the friends, sometimes that of the headquarters (-praetoriani-), had the duty of serving in the latter (-praetorium-) in return for which it was exempt from camp and entrenching service and enjoyed higher pay and greater repute.
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Reference address : https://www.ellopos.net/elpenor/rome/4-06-marius-revolution-drusus-reform.asp?pg=12