In the 1979 script by Coppola and Puzo the antagonist was a New York don. This may have been the predecessor of the Don Altobello character. I think he was a little too prominent within Mafia circles in New York to just be a don from Sicily as he was the one who set up the meeting of the Commission. Remember the line "as your family's oldest friend I'm always chosen to bring you messages". I doubt the other New York dons would use a Sicilian don to bring messages to Michael. He could have been the Don of the Tattaglias, but then he must've already been close to Vito before Philip Tattaglia became don.