In '79 when AD&D is a completed ruleset, you get an introductory product.
Albeit one that wasn't designed for AD&D, despite Gygax's retrofitting of the rules and text to make it seem so.
Looking back on the early releases of OD&D and AD&D, and then taking into account this
interview with Rob Kuntz, I become more awed at Gary Gygax's achievements. To a large extent, you have TSR flailing about and trying to work out what it has. (It's worth comparing to Wizards when they first published Magic; the first expansion was planned to have
different coloured backs!, only to be changed at the last minute. The first several sets are weakly developed in comparison to what came later).
Original D&D is "make up your own stuff", very much so. AD&D combines the disparate expansions to OD&D into a semi-coherent whole, but the motivations are confused further by the TSR-Arneson feud. (I'd say Gygax-Arneson, but when I read posts by Tim Kask, I expand it to TSR).
If D&D had remained as RJK wanted it, it'd be dead now.
Part of Gygax's genius was recognising things that worked and incorporating them into the master plan.
"The actual philosophical change occurs when someone, I forget whom, sent Gary Gygax a copy of a pre-made adventure, Palace of the Vampire Queen. Many of us looked at it—I even picked up a copy for myself-- in a mode of perplexed inquiry. The majority of us were vocal about why anyone would want someone else creating things for them and their campaign worlds whereas all of the resources in primary and supportive categories were available to them to create their own material." - RJK
From here, TSR move to produce adventures. There's a small gap between the 1976 printing of PotVQ and the 1978 printing of G1. The original ones on hand are the tournament adventures for Origins (G1-3), and - if I have it correctly - they premiered for sale at the convention. The gap is probably mainly because TSR is so amazingly small. For the largest part, we're talking mainly about Gary Gygax when it comes to D&D - and he's somewhat distracted pulling together the notes for AD&D, as well as running TSR.
In the early history of D&D, there are three figures that stand out for me:
Gary Gygax, for creating a large part of it and shepherding the game into its AD&D form and its initial adventures.
Tom Moldvay, for editing the Basic D&D set of 1981, which brought the game into a form that new players could understand.
Tracy Hickman, for showing what was possible with the adventure module.
Cheers!