It's the first appearance of Schroeder's famous bust of Beethoven! Also, the first time he's said "Beethoven." It's fun to say Beethoven. Beethoven!
Technically that bust breaks the rules about depicting adult figures, but it is just a knickknack, and it's nice to see that Charles Schulz could render realistic faces too. There's so much character in that face. I think half the humor in this one comes from the different art style used to render that bust.
It seems to me that, over time, the characters get bigger. I think it comes from the slightly more mature proportions and the decreasing thickness of the lines. There's usually nothing to compare scale with other than the other characters, but Schroeder's piano and Beethoven bust give us something to judge scale by. Here the bust is bigger than the piano, and juts out over the top. Lucy wouldn't have any room to lean here. Later on the bust fits entirely on the piano, implying that either the bust is smaller or the piano is bigger.
