Interesting question, Oli. We see Vito with the Commission in GF, and Michael with them in AC in III. But, there's no Commission in II, even though we know that the Commission existed in the Fifties and met (famously, in Apalachin NY, in '57). My guess is that Michael, obsessed with his "legitimacy" in Nevada, would never have attended a Commission meeting for fear of exposing his Mob-ness. He probably had Pentangeli sit in for him, as Gigante had Fat Tony Salerno sit in for him while The Chin was pretending to be non compos mentis. Of course, that begs the question of why Michael would preside over a Commission meeting in III, when he was even more "legitimate." But then, it's one of many things about the plot of III that are illogical.