Peanuts by Charles Schulz for February 22, 1961
Transcript:
Charlie Brown and Linus walk to school carrying their lunch bags. Linus says, "I always enjoy reading the little notes my mother puts in my lunch . . ."<BR><BR> Linus looks at the note and says, "Hmm . . . So now she's a comedienne . . ."<BR><BR> Charlie Brown says, "Why? What does she say?"<BR><BR> Linus reads, "'Help! I'm being held prisoner in a school lunch factory!'"<BR><BR>
No, I think Schulz was referencing an existing joke. However, I once really did get a message saying: “Help! I’m being held prisoner in a Chinese bakery!” I nearly died laughing. :-)