This is the first real sign of Lucy's loud, angry persona. Not in what she does; she's been cranky before. The key element is how Charlie Brown recognizes it, and Patty reacts to it in the last frame.
The letters of the word BANG here appear to be stenciled or pasted, or at least drawn with mechanical aid, instead of hand-drawn. I wonder if that was how they were originally rendered or if the syndicate replaced them?
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