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
According to Sulla's arrangement Italy proper, the northern boundary of which was at the same time changed from the Aesis to the Rubico, was--as a region now inhabited without exception by Roman citizens--made subject to the ordinary Roman authorities; and it became one of the fundamental principles of Roman state-law, that no troops and no commandant should ordinarily be stationed in this district. The Celtic country south of the Alps on the other hand, in which a military command could not be dispensed with on account of the continued incursions of the Alpine tribes, was constituted a distinct governorship after the model of the older transmarine commands.(27)
27. For this hypothesis there is no other proof, except that the Italian Celt-land was as decidedly not a province--in the sense in which the word signifies a definite district administered by a governor annually changed--in the earlier times, as it certainly was one in the time of Caesar (comp. Licin. p. 39; -data erat et Sullae provincia Gallia Cisalpina-).
The case is much the same with the advancement of the frontier; we know that formerly the Aesis, and in Caesar's time the Rubico, separated the Celtic land from Italy, but we do not know when the boundary was shifted. From the circumstance indeed, that Marcus Terentius Varro Lucullus as propraetor undertook a regulation of the frontier in the district between the Aesis and Rubico (Orelli, Inscr. 570), it has been inferred that that must still have been provincial land at least in the year after Lucullus' praetorship 679, since the propraetor had nothing to do on Italian soil. But it was only within the -pomerium- that every prolonged -imperium- ceased of itself; in Italy, on the other hand, such a prolonged -imperium- was even under Sulla's arrangement--though not regularly existing--at any rate allowable, and the office held by Lucullus was in any case an extraordinary one.
But we are able moreover to show when and how Lucullus held such an office in this quarter. He was already before the Sullan reorganization in 672 active as commanding officer in this very district (p, 87), and was probably, just like Pompeius, furnished by Sulla with propraetorian powers; in this character he must have regulated the boundary in question in 672 or 673 (comp. Appian, i. 95). No inference therefore may be drawn from this inscription as to the legal position of North Italy, and least of all for the time after Sulla's dictatorship. On the other hand a remarkable hint is contained in the statement, that Sulla advanced the Roman -pomerium- (Seneca, de brev. vitae, 14; Dio, xliii. 50); which distinction was by Roman state-law only accorded to one who had advanced the bounds not of the empire, but of the city--that is, the bounds of Italy (i. 128).
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Reference address : https://www.ellopos.net/elpenor/rome/4-10-sullan-constitution.asp?pg=37