When you look at the later episodes that feature Magnum's suspicion of Higgins/Robin, particularly the scene in "Paper War", it's clear that Magnum believes there is a person named Robin Masters who has jet-setted around the world drinking Lafite Rothschild and eating caviar. Indeed, Magnum has met him several times and was standing right next to him in "The Big Blow"! But the person with all the power and money, the person who actually wrote all those tawdry pulp novels, the person who owns all those properties (including Robin's Nest) is Jonathan Quayle Higgins III! Magnum correctly deduces that Higgins is pulling all the strings. He had to create this facade so his family name wouldn't get sullied.
Consider this key passage from "Paper War":
Higgins: What on earth are you talking about?
Magnum: The big lie Higgins, admit it! You're Robin. YOU are Robin Masters!
Higgins: I-I'm laughing at the sheer absurdity of the accusation.
Magnum: No, no, no, you're laughing, because you're trapped. YOU have NEVER laughed like this. Now admit it. You've spent all these years, pretending to be Robin's employee, because you didn't want anybody to know that you write cheap pulp novels.
Higgins: (laughing wildy) And who, may I ask, is the man we know and address as Robin Masters?
Magnum: I don't know, some little guy with a voice like Orson Welles and a body like Truman Capote that you hired to pose as Robin. And it was very interesting casting. You weren't satisfied with the nom de plume. You developed this whole persona, to create the kind of playboy you envisioned writing cheap pulp, so YOU could devote yourself to serious writing.
As far as the yearbook thing goes..... that's a tough one to explain away. Plastic surgery, maybe?
