Commentary Inf VI 79-81
If there are only two just Florentines in the city in 1300, in 'the good old days' there seem to have been at least five, men who made great effort to offer sustenance to goodness. The meaning of the tercet is clear enough. The problem arises because one of the five, Arrigo, is never mentioned -- or surely does not seem to be. The other four are seen in Hell, Farinata in canto X, among the heretics, Tegghiaio and Iacopo in XVI, among the Florentine homosexual politicians (the most positively presented sinners since those we met in Limbo), and Mosca, in a sense the only 'surprise' here, since he is punished in canto XXVIII for his treachery on the battlefield, but, in Dante's mind, is apparently considered, along with Farinata and the others named here, as among the powerful citizens of an earlier Florence, before the city became utterly dissolute, who maintained a proper civic concern. See the study of Pietro Santini (Sant.1923.1) for this argument, which is espoused by Mazzoni (Mazz.1967.2), pp. 46-48.

Of course, it is the puzzle created by Arrigo's not being further referred to in hell that has drawn commentators like flies to rotten meat. For a review of the many competing 'Harrys' who began to populate the margins of Dante's poem in the fourteenth century and have continued to do so into our own, see Vincenzo Presta, 'Arrigo' (ED.1970.1), pp. 391-92. Presta prefers the candidacy, first advanced by Santini, of one Arrigo di Cascia, as does Mazzoni (Mazz.1967.2), p. 47. All one can say is that it is strange that Dante picked the names of five men, four of whom he goes on to include prominently in his poem, and one that he does not mention again -- especially since he has Ciacco say explicitly that the protagonist will see all of them in his descent ([Inf VI 87]). This is a mystery that will probably always remain a mystery.