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
Inasmuch as the law conferred beforehand on the twenty-five adjutants whom he was to nominate praetorian rank and praetorian prerogatives,(10) the highest office of republican Rome became subordinate to a newly created office, for which it was left to the future to find the fitting name, but which in reality even now involved in it the monarchy. It was a total revolution in the existing order of things, for which the foundation was laid in this project of law.
10. The extraordinary magisterial power (-pro consule-, -pro praetore-, -pro quaestore-) might according to Roman state-law originate in three ways. Either it arose out of the principle which held good for the non-urban magistracy, that the office continued up to the appointed legal term, but the official authority up to the arrival of the successor, which was the oldest, simplest, and most frequent case. Or it arose in the way of the appropriate organs--especially the comitia, and in later times also perhaps the senate--nominating a chief magistrate not contemplated in the constitution, who was otherwise on a parity with the ordinary magistrate, but in token of the extraordinary nature of his office designated himself merely "instead of a praetor" or "of a consul."
To this class belong also the magistrates nominated in the ordinary way as quaestors, and then extraordinarily furnished with praetorian or even consular official authority (-quaestores pro praetore- or -pro consule-); in which quality, for example, Publius Lentulus Marcellinus went in 679 to Cyrene (Sallust, Hist. ii. 39 Dietsch), Gnaeus Piso in 689 to Hither Spain (Sallust, Cat. 19), and Cato in 696 to Cyprus (Vell. ii. 45). Or, lastly, the extraordinary magisterial authority was based on the right of delegation vested in the supreme magistrate. If he left the bounds of his province or otherwise was hindered from administering his office, he was entitled to nominate one of those about him as his substitute, who was then called -legatus pro praetore-(Sallust, lug. 36, 37, 38), or, if the choice fell on the quaestor, -quaestor pro praetore- (Sallust, Iug. 103).
In like manner he was entitled, if he had no quaestor, to cause the quaestorial duties to be discharged by one of his train, who was then called -legatus pro quaestore-, a name which is to be met with, perhaps for the first time, on the Macedonian tetradrachms of Sura, lieutenant of the governor of Macedonia, 665-667. But it was contrary to the nature of delegation and therefore according to the older state-law inadmissible, that the supreme magistrate should, without having met with any hindrance in the discharge of his functions, immediately upon his entering on office invest one or more of his subordinates with supreme official authority; and so far the -legati pro praetore-of the proconsul Pompeius were an innovation, and already similar in kind to those who played so great a part in the times of the Empire.
Do you see any typos or other mistakes? Please let us know and correct them
Reference address : https://www.ellopos.net/elpenor/rome/5-03-fall-oligarchy-pompeius.asp?pg=28