 In the episode "Trick or Treat", Skull has the numbers "24601"  tattooed on his chest. This is reference to Jean Valjeans prison  number in Les Miserables (also used by Seymour Skinner and Sideshow  Bob in _"Simpsons, The" (1989)_ (qv), Hank Jennings in  _"Twin Peaks" (1990)_ (qv).