Hello Ludovigo,
What source of Apostolo and Massimello are you using?
In the book, ITALIAN ACES OF WW2, by Apostolo and Massimello, the chapter that is written about Ferrulli states ".....with a few 91 Sq. veterans surviving its numerous campaigns, combat accounts from the unit are hard to find. This lack of information is further compounded by the near-total destruction of the unit's official wartime records. Therefore, when it came to recounting Ferrulli's career only fragments of information were available, which was cross-checked with accounts frome those with the personal knowledge of his achievements..."
It is very difficult to try and match dates with claims, since the 4th Stormo's records were lost during the Sicilian campaign.
You might want to try Hakans Aviation page.
He does a fantastic job with the Bi planes and Aces of WW2.
http://surfcity.kund.dalnet.se/index.html
Vinman.