This is the first strip in which Schulz draws complex music notation, which he said in Peanuts: A Golden Celebration he found challenging yet satisfying to draw. Schroeder's still playing the generic baby, so the two haven't met yet.
I remember that there is a similar early Garfield strip along these lines. Garfield and Odie sing with each other. Jon comes along singing something different, represented by a different note. In the last panel, the animals shout their note at him while Jon reacts in surprise. It's one of the more entertaining early Garfield strips, I seem to remember.
