Flintstones

In 1977, Marvel inked a deal allowing them to publish comics featuring Hanna-Barbera cartoon characters. The Flintstones was one of the first of these series (Others were "Dynomutt," "The Funtastic World of Hanna-Barbera," a treasury-sized series, "Scooby-Doo," and "Yogi Bear.") and, as such, presented a feature page showing Spider-Man shaking hands with Fred Flintstone. Spidey's hand is also featured holding onto the Bullpen letter introducing the Marvel-HB deal and Spidey's face is on a stamp pasted on an envelope apparently waiting for the insertion of the Bullpen letter.