For many Star Wars fans, "THX 1138" is the ultimate inside joke - a subtle nod from the filmmakers to audiences in the know. In A New Hope, when Han Solo and Luke Skywalker are infiltrating the Death Star detention cell, Luke explains Chewbacca's unusual presence as a "prisoner transfer from cellblock 1138." In the original shooting script, the reference was a little bit more veiled -- it was cellblock TS-138. The novelization of Star Wars uses this earlier version, but makes up for it by replacing Luke's stormtrooper identifier used in the film. Instead of asking why TK-421 isn't at his post, the Imperial gantry officer asks "THX-1138, why aren't you at your post?"
In Empire, the THX reference is a little bit more sly. When the Rebels discover the possible presence of droid intruder on Hoth, General Rieekan gives the following orders: "Send Rogues 10 and 11 to Station three-eight."
Return of the Jedi puzzled Easter Egg-finders for over two decades now. Many fans assumed that the inclusion of a THX in-joke is a priority inclusion, a line item on some unseen production checklist that must be crossed off in order for the film to be completed. In reality, the in-joke only happens if someone remembers to include it. If there is one in Jedi, it has yet to be found, despite years of fan dissection and discussion. The closest thing yet found is the name of a slots-playing background droid BG-J38. The "BG-J" stands for "Background - JEDI," but where does the 38 come from? Very vague and unsatisfactory, but that may be it for a THX reference.
UPDATE: Since this article was published in 2004, a hitherto unknown THX 1138 reference was discovered by a licensee producing a Boushh collectible. By closely examining reference photography of the physical helmet, a faint 1138 can be seen painted on the side of the bounty hunter headgear. Amazingly, there are still secrets being uncovered from these films after so many years.
The Phantom Menace made up for Jedi's nebulous reference by putting its THX gag more front-and-center. It's on the back of a shut down battle droid after the end of the Naboo plains battle.
Attack of the Clones has an impossible-to-find reference, but those involved in the post-production of Episode II know it's there. On the back of each digitally-created clone trooper's helmet is a tiny LED that displays the soldier's identifying number. These are completely illegible on the big-screen, but the Viewpaint artists have nonetheless painted a number in there - each clone is designated "THX 1138."
Though the final edit of Revenge of the Sith is still evolving, the THX reference is very apparent in the script to Episode III. Clone Commander Bacara, serving Jedi General Ki-Adi-Mundi on a distant world, is identified in the script as Commander 1138.
