Bonanno says nothing about his rank prior to his elevation. But he does say that during the war he acted as Mranzano´s chief of staff. Without mentioning any details, he also says that he didn´t attain that position by being a spectator. In that capacity, I suppose Bonanno was Maranzano´s right hand man (but not official underboss) taking care of stuff that needed to be done in order to win the war. As much as he was a gunner, he was also a bodyguard together with a group of men who called themselves "The boys of the first day". Because of the war, traditional ranks within the Family were not important. Keep in mind that the official boss Cola Schiro had vanished, the second most important member Vito Bonventre had been killed and Joe Parrino, another powerful member had been appointed boss by Masseria, but ostracized by his own group. All this put the Castellammarese in a disarray. We don´t know Bonannos rank before the war and after the dust had settled. But he was most likely an ordinary soldier. When Maranzano came out victorious, we know that Angelo Caruso was made Maranzano´s official underboss holding that position until Bonanno took over the Family. Also keep in mind that Stefano Magaddino, Bonanno´s cousin and the most powerful Castellammarese after the death of Maranzano, in a bid to control the NY Castellammarese, pushed Bonanno into the top position. In that regard, Bonanno´s climb from a mere soldier to boss sounds plausible. The Bonanno name carried a lot of weight within the Castellammarese comunity around this time. This must not be overlooked in my opinion.