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
Although the numerical proportions of the burgesses and non-burgesses in Italy cannot be properly ascertained, it may be regarded as certain that the number of the burgesses was not very much less than that of the Italian allies; for nearly 400,000 burgesses capable of bearing arms there were at least 500,000, probably 600,000 allies.(7)
7. These figures are taken from the numbers of the census of 639 and 684; there were in the former year 394, 336 burgesses capable of bearing arms, in the latter 910,000 (according to Phlegon Fr. 12 Mull., which statement Clinton and his copyists erroneously refer to the census of 668; according to Liv. Ep. 98 the number was--by the correct reading-- 900,000 persons). The only figures known between these two--those of the census of 668, which according to Hieronymus gave 463,000 persons-- probably turned out so low only because the census took place amidst the crisis of the revolution.
As an increase of the population of Italy is not conceivable in the period from 639 to 684, and even the Sullan assignations of land can at the most have but filled the gaps which the war had made, the surplus of fully 500,000 men capable of bearing arms may be referred with certainty to the reception of the allies which had taken place in the interval. But it is possible, and even probable, that in these fateful years the total amount of the Italian population may have retrograded rather than advanced: if we reckon the total deficit at 100,000 men capable of bearing arms, which seems not excessive, there were at the time of the Social War in Italy three non- burgesses for two burgesses.
So long as with such proportions the burgesses were united and there was no outward enemy worthy of mention, the Italian allies, split up into an endless number of isolated urban and cantonal communities, and connected with Rome by a thousand relations public and private, could never attain to common action; and with moderate prudence the government could not fail to control their troublesome and indignant subjects partly by the compact mass of the burgesses, partly by the very considerable resources which the provinces afforded, partly by setting one community against another.
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Reference address : https://www.ellopos.net/elpenor/rome/4-07-sulpician-revolution.asp?pg=8