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
Stage of Development of the Celtic Civilization
If we survey the whole condition of the Celts as Caesar found it in the Transalpine regions, there is an unmistakeable advance in civilization, as compared with the stage of culture at which the Celts came before us a century and a half previously in the valley of the Po. Then the militia, excellent of its kind, thoroughly preponderated in their armies;(23) now the cavalry occupies the first place.
23. Cf. II. IV. The Celts Assail the Etruscans in Northern Italy
Then the Celts dwelt in open villages; now well- constructed walls surrounded their townships. The objects too found in the tombs of Lombardy are, especially as respects articles of copper and glass, far inferior to those of northern Gaul. Perhaps the most trustworthy measure of the increase of culture is the sense of a common relationship in the nation; so little of it comes to light in the Celtic battles fought on the soil of what is now Lombardy, while it strikingly appears in the struggles against Caesar.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=40