|Lot of early Wild Hunt APAzines, one of which is issue #19, from an Ebay auction|
"In time for the third incarnation of the Origins convention - this time beginning July 22 in Staten Island, New York, under the stewardship of SPI - TSR produced the Basic Set of Dungeons & Dragons" (pg 579-580) and "TSR promoted the Basic Set heavily at all of the 1977 summer conventions, beginning with Origins III" (pg 581).
There's no clear source for these particular statements in his book, but Jon kindly sent me some quotes from Bill Seligman's report on Origins III in The Wild Hunt #19 (Aug 1977):
"The new D&D is out, or at least, the first book of it. It's called 'Basic D&D' and it contains enough information to set up three Gygaxian D&D levels. The book itself costs five dollars, and contains new spells, better writing. (it was re-written by John Eric Holmes, who wrote some sequels to ERBs Pellucidar series) and more examples. There are still several errors - or are there? Some errors may be deliberate holes left for the advanced D&D rules to fill."
Jon comments that "Especially from this last sentence, it seems clear that he had seen a copy and had time to read it, not just heard that it was out ... Seligman was also aware of the boxed set."
The Wild Hunt was, like Alarums & Excursions, an Amateur Press Association fanzine (APAzine). It was started in 1975 by editor Mark Swanson, based in Massachusetts, and lasted until the early '90s. In their time the APAzines served much the same role as blogs do today, allowing amateur writers to share their thoughts. The editor would bundle the contributions of each writer, which could be anything from new character classes for D&D to movie reviews, and send out a copy to each contributor.
Thanks to Jon and Mr. Seligman we now have strong evidence that the Basic Set was available as early as July 22nd, 1977, which is actually pretty close to the July 10th date of the copyright record.