The newspaper article was entitled "Gilbert and Sullivan Clue to Zodiac". Therefore, it shouldn't be any surprise that the Zodiac Killer chose a greeting card that opened with the wording "From your secret pal. I feel it in your bones, You ache to know my name, And so I'll clue you in". This was the only communication thus far that contained white text, which the Zodiac Killer adopted by adding 4-TEEN, BOO and the symbolim at the foot of the card, rather than using pasted newspaper clippings. The Zodiac Killer was very likely claiming 14 victims when he wrote this total on the skeleton's hand on the front of the card, followed by a head count of 4-TEEN above the second skeleton on the card inner, and concluding with the symbolism at foot of the card.
"Peekaboo (also spelled peek-a-boo) is a form of play played with an infant. To play, one player hides their face, pops back into the view of the other, and says Peekaboo!, sometimes followed by I see you! There are many variations: for example, where trees are involved, "Hiding behind that tree!" is sometimes added. Another variation involves saying "Where's the baby?" while the face is covered and "There's the baby!" when uncovering the face". Wikipedia. A British term for "peek-a-boo" is "peep-bo" - which just happens to be one of the characters from The Mikado.
The Zodiac Killer chose the introduction on the Halloween card outer in response to the newspaper article entitled "Gilbert and Sullivan Clue to Zodiac", because it stated "And so I'll clue you in". He then responded to this introduction by inserting "peek-a-boo" around the knothole of a tree, with an eye peering from within. A saying which has the alternative of peep-bo - a character from the Gilbert and Sullivan opera. The teaser of "I'll clue you in" and the answer of "peep-bo" both inspired by The Mikado featured in the October 12th 1970 newspaper article. The Zodiac Killer would not give us the Ko-Ko investigators were desperately attempting to track down, only the protector or ward of Ko-Ko.