I remain open to there being a unified play culture around Gygax, TSR, and his supporting institutions (the Dragon, the RPGA, post-1977 Judges Guild, other suggestions welcome) that was actively cultivated from 1977-1982. I agree with Prince that textual production is an important historical indicator for tracking this stuff's development and progression.
If this culture exists/existed, I'd use the term "classic" to distinguish it from "old school" which has too many other prominent associations to do more than confuse. "Classic" would be the only culture of play that didn't have an autonym (tho' I think / hope the positive associations of "classic" would appeal to its adherents as a self-designation).
One important point of distinction between "classic" and the "OSR" would be that Gygax saw progressively escalating challenges balanced to progressively escalating PC agency as a core dynamic of play expressing a value of "fairness" or "balance" (he even uses the dreaded term "game balance" to defend specific game design decisions). OSR design tends to be more interested in variance in PC agency for its own sake. I tend to consider this distinction to support the existence of classic as a distinct culture.
For an end-date of 1982 for the active institutional spread of this culture:
1) The RPGA seems to support and advance "classic" play at conventions in 1981 and 1982, but not from 1983 onwards. I spent the weekend digging through Polyhedron and the RPGA modules from 1981-1983 at the suggestion of a friend that tournament modules from the RPGA would have been an important contemporary vector for spreading "classic" play styles. The RPGA modules in 1981 ("To the Aid of Falx", etc.) are written by Frank Mentzer and are definitely not trad. If one believes "classic" exists, then they are definitely expressive of its proposed values.
1982's RPGA modules are a mixed style tending to classic nonetheless, with "Investigation of Hydell" being fairly pure classic, but "Egg of the Phoenix" showing strong trad influences without fully expressing the style. The adventure opens up with a page of canned description that includes a narrated conversation a PC pre-gen has with the NPC quest-giver but then turns into a series of challenges with only a loose story. In 1983, Tracy Hickman (boo! hiss!) takes over writing the modules and they become expressive of his trad values. 1984 and 1985's module series have different writers, but are pretty clearly trad as well (To Find a King and all that).
2) 1982/1983 is also when Gary becomes mostly hands-off on the actual gaming side of TSR, loses a lot of control of the company to the Blumes, divorces his wife, and goes to Hollywood to try to drum up interest in D&D. This comes to a head in 1983, when the actual divorce and move happen, but from what I can tell, his collapsing marriage, drug use, and management fights with Brian Blume occupied most of 1982.
Gygax's only publications in 1982 are a republication of Lost Caverns of Tsojcanth (actually written five years beforehand), and Forgotten Temple of Tharizdun. FTT famously has art by Karen Nelson (a friend of Gygax's) because he was already being stymied in dealing with the design department at TSR by Blume.
1982 is a very good year for Mentzer, Moldvay, and Douglas Niles (obvious candidates for authors working with a "classic" set of play culture norms who were still publishing new texts through TSR) in terms of publications, but Moldvay would mostly stop publishing through TSR after 1982 (he shifts to Lords of Creation in 1983, which I think of as traddish in orientation). Douglas Niles became a trad guy from corresponding and co-creating Dragonlance with Hickman. Mentzer seems to have held out for Gygax's values for a while, but when you look at the republications of his "classic" modules (e.g. the I12 republication of Egg of the Phoenix) that he oversaw, they're definitely revised to better correspond with trad norms and values, and much of his later work (even with Gygax, e.g. Dangerous Journeys) seems trad-oriented.
3) The Judges Guild, which I agree settles down into a firm supporter of Gygax's style sometime in 1977, but has its license to release officially-branded material for AD&D in 1982. By 1985, the company is basically out of business (until revived later).
And so on and so forth.
The end of active institutional support for the culture doesn't mean it stopped existing (if, indeed, it ever existed), but it does explain how trad became hegemonic despite it being preceded by "classic" play.
For me, the next phase will be looking at publications outside of TSR's immediate influence (JG is 3rd party but working under a licensing deal) for people echoing Gygax's norms autonomously. I would consider texts that did so to be strong evidence in support of the existence of that culture. Recommendations are, of course, appreciated.