Ralph Bellamy is the actor who plays Bruce Baldwin. But he's also an inside joke. When Louie asks Walter what Bruce looks like, he responds,
WALTER: That guy in the movies, Ralph Bellamy.
The joke isn't just that Bruce is Ralph Bellamy. It's that he is the kind of guy Ralph Bellamy would play. Bellamy played a similar role to Bruce in the film The Awful Truth — which Cary Grant also starred in. Just as in His Girl Friday, Grant's character divorced his wife, who then took up with a Midwestern innocent played by Bellamy.
The filmwriters are therefore winking at the audience, letting everyone know that they know about the earlier film. But it's not just the screenwriters doing the winking—it's Walter.
Walter's so smart, he knows he's in a movie. He's so sophisticated, he knows that Ralph Bellamy plays (and is playing) Midwestern bumpkins. The actor, Ralph Bellamy, becomes a sign of Walter's smarts—all without poor Bruce ever knowing.