We would have to wait until 1979 to see a Beholder appear in a published adventure. That adventure was "The Fell Pass", which was published in The Dragon magazine, Issue #32, in December 1979.
The Fell Pass is a dungeon crawl. It reminds me (a little) of the episode from The Hobbit, where the Dwarves are captured by the Goblins, and Bilbo first encounters Gollum. Since it is a mountain "pass", the party may enter the dungeon from either side, and make (or fight) their way through to the other side.
The Fell Pass is interesting, as it can be played simply as an attempt to make it from one side of the pass to the other, without dying. Alternately, you can also treat it as your standard dungeon crawl, or have a patron assign the cleansing of the pass to the party. In any event, there are no over-arching themes, no BBEG to kill, just some good old-fashioned tricks and puzzles, and mindless hack-n-slashery.
That is not to say there are no challenging adversaries or mind-blowing dangers in The Fell Pass. It is a dangerous module, and was probably informed by the other early modules of the day, including that most infamous of killer-dungeons, The Tomb of Horrors.
As for the Beholder, well, let's just say that Xorddanx is a wily and crafty opponent, and the Players will have a difficult time defeating him, as he uses the terrain and his followers to great advantage.
While The Fell Pass is not an "official" D&D module (you will not find The Fell Pass on any list of officially published adventures) it certainly could have been, it has that early D&D module quality and feel to it.