In fact, "The Thief Addition" had first shipped in a fanzine called the Great Plains Games Players Newsletter back in May. Few saw the rules there, however, as the circulation of the zine at the time was only a couple dozen copies. Gygax invited the zine's editor, Jim Lurvey, to sell copies of the issue with "The Thief Addition" at the TSR booth; unsurprisingly, Lurvey reported that sales "went fast." (As an aside, Lurvey also sold the next issue there, which was the first place to feature Gygax's rules for a percentile Strength modifier.)
One very important thing to glean from the two pages worth of rules sandwiched between official-looking title pages is that Gygax gives us some backstory about the development of the Thief class. He adopted the idea from a fan who he spoke to over the fan; he gives the name here as Gary Schweitzer, but most likely he meant Gary Switzer, who played with the Aero Hobbies crowd in Santa Monica, California (see Section 5.2 of Playing at the World for a bit more context about Switzer). On the basis of that conversation, Gygax developed his own preliminary version of the Thief, which he stresses have "not been tested." The testing presumably would come at GenCon.
As to the rules themselves, they evince most of the key skills that Greyhawk, and innumerable later role-playing games, would associate with Thieves. They can open locks, remove traps, climb walls, steal, backstab, hide in shadows, listen for noises and move silently. These skill checks are mostly resolved with a percentile roll, a precedent that would also have a profound influence on later game designs. As areas of "other possible consideration," Thieves of sufficient level may decipher foreign languages and even cast Magic-user spells from scrolls. Small particulars differ from Greyhawk: the way that backstab damage is handled, for example. But the fundamentals are all in place.
Even thirty-eight years ago, GenCon was a place to show off your new ideas and get feedback, and a place where you could bring home a glimpse of the future of gaming. I wonder what this year will have in store for us...