Like many others here, I never saw Frankie Five Angels as being stupid. Coarse, perhaps, and without the veneer of the graceful (yet hypocritical) socialite, for sure. But he wasn't really stupid.

He was the successor to Clemenza, and truly stupid people do not make it to the level of caporegime. He was a street guy, yes, and he probably even felt a certain degree of disdain for those who pretend to be somehow "better" than a street soldier. I have known men who shunned the pretensions of self-styled intellectuals and socialites altogether and who knew that this was what they were doing. They did so intentionally, sometimes to hide their true shrewdness behind a clumsy facade-- and at other times in an effort to stage a subtle rebellion against others whom they saw as phonies.

I think that he was cut from the same bolt of cloth as his former boss, and that his true strength was of a more tactical, less strategic, sort of outlook. He himself told Michael, "I don't -- look -- I don't have your brain -- uh -- for big deals -- but this is a street thing." But then he shows a great deal of insight and awareness by following this declaration up with the statement, "That Hyman Roth in Miami -- he's backing up those son of a bitches."

I would respectfully submit that these are not the words of some ignorant buffoon. Frankie was razor-sharp, and never mind his rough-hewn mannerisms. As far as the canapes question is concerned, I think he was just making a very clever and extremely funny joke... a wonderful play on words.

He probably knew all about the Romans because the subject interested him and because it was a matter of pride and heritage to him. He might have faltered if you asked him a question about Kafka (while the likes of Crazy Joe Gallo could have remarked intelligently about this subject, which apparently interested him), but no, Frankie was no fool.

For sure, Frankie could have a guy killed with relative ease. Look at how old he was before he was ever nailed on anything substantial-- and only after being the primary informant against himself-- yet, we know for sure that he was a killer and a really tough old bird who had been operating for years.

I always liked Frankie and Pete Clemeza better than even Vito Corleone, and way better than I ever liked Michael. Their ambitions might not have been so lofty, but they were more true to their original code.

Near the end of his life, Vito rambled to Michael about not being subject to the whims of "those big shots", while apparently overlooking the fact that he was himself a big shot. But Vito and Michael seemed to need something more than just the honor (and loot) they received by being a part of their regime. They aspired towards political power and a sort of overlordship that I don't think Clememza or Frankie were ever interested in.

All of which is to say, in answer to dontomasso's original question:

I think that Frankie was indeed the shrewd underboss we saw at the end. And I think he was also a shrewd underboss at the very beginning of the movie too... we just didn't have a chance to understand this about him yet because he hadn't been fully developed as a character at that point in the film.