So I've been working on the essay, and specifically elaborating the proto-culture / pre-trad piece. As background, I spent the past week refreshing myself by rereading:
1) OD&D white box
2) Empire of the Petal Throne
3) Boot Hill
4) Greyhawk, Blackmoor, Eldritch Wizardry
5) City State of the Invincible Overlord
6) The Strategic Review's entire run / The first two years of Dragon
7) The Judge Guild's Journal
8) The 1977 Holmes Basic set
9) The 1979 AD&D 1e DMG
So one of the interesting things about the OD&D Whitebox is how much of the instruction about what one can do with it is implicit. For example, it never describes what the point of the game is, and the guidance on using the material is mainly restricted to advising one to starting off with a dungeon of a certain size. Even a straightforward statement like "This is a game where you take on the roles of individual characters exploring ruins with the hope of recovering treasure while braving perilous foes" isn't in it.
EPT, similarly, gives you a lot of ideas of things you could do, and even a basic campaign premise (the PCs are recent immigrants looking to move up in social status through taking on missions) but leaves ideas about what these missions could be mostly implicit, outside of some suggestions of the high politics of the empire.
It's actually Boot Hill in 1975 that first contains an explicit statement of what one is supposed to be doing in the game, in its foreword and introduction.
"These rules are aimed at enjoyment on a plane unusual to wargaming, the individual and personal. Rather than commanding hordes of troops players typically have but a single figure, their "character". With these figures the players recreate the individual gunfights, saloon brawls, and Wild West action as has come down to us from the pages of history — and the celluloid of Hollywood Westerns."
and
"The players then go about the actions appropriate to their chosen roles in the game — or the roles assigned to them by the game referee — more or less letting the nature of the Wild West take its course. Within a turn or two things begin to happen, and before long all hell has broken loose... These games can be played as single events, each unrelated to the next except for the "experience" which the characters might have gained or the substitution of a new personal figure due to the incarceration or demise of a former one during the course of the previous game. It is better if games are strung together as an epic of action, with the whole taking place in some general locale and past happenings being reflected more directly in each successive game"
The introduction also suggests that Boot Hill can be played with or without a referee and "While it is possible to structure rigid scenarios — in the manner of the two included herein — we suggest that free-form play will usually prove more interesting and challenging. Set up a town, give a few background details, and allow the participants free-rein thereafter."
That's basically it. There are some interesting similarities to D&D obviously, but this is not by any means a statement of what D&D is about or how to use its rules.
Greyhawk, Blackmoor, Eldritch Wizardry, and City State are all similarly silent. Really, Gygax's first address to the public of what the game is about is in the April 1976 issue of the Strategic Review, where there are a number of articles dealing with facets of the game and along the way he begins to elaborate a specific idea of what D&D is about.
In that issue, in his article "The Dungeons and Dragons Magic System", he says "While miniatures battles on the table top were conceived as a part of the overall game system, the major factor was always envisioned as the underworld adventure, while the wilderness trek assumed a secondary role, various other aspects took a third place, and only then were miniatures battles considered."
Later in this same article Gygax mentions that there are people who are already drifting away from his vision of D&D.
There's also a column towards the end of the issue called "D&D is Only As Good as the DM" that emphasises Gygax's own beliefs that challenge is central to the game (and that creating and managing these challenges is explicitly the job of the DM) - while also specifically calling out another crew of players who aren't playing the game the way he envisioned:
"Now I know of the games played at CalTech where the rules have been expanded and changed to reflect incredibly high levels, comic book characters and spells, and so on. Okay. Different strokes for different folks, but that is not D & D. While D & D is pretty flexible, that sort of thing stretches it too far, and the boys out there are playing something entirely different — perhaps their own name “Dungeons & Beavers,” tells it best."
He also complains that no one can have reached higher than level 14 yet according to his vision of how the game is to be played because no one has done so yet in Blackmoor or Greyhawk, the oldest campaigns that follow that vision.
So this seems pretty clear to me that even by 1976, we're already having people abandon the progressive, challenge-oriented gameplay and campaign structure Gygax envisioned but did not articulate explicitly in OD&D.
Interestingly, the Judges Guild Journal in its first issue (October 1976) is already talking about "plots" and "dramas" tho' these are mostly exhortations that are taken as self-explanatory. In general, reading through the journal's run in the late 1970s, you get Bledsaw and Holsinger elaborating very very different takes on D&D from anything Gygax is doing (even tho' Gygax signs off on a lot of it whenever they see him at GenCon and ask him about it). Bledsaw elaborates in a latter issue that he was introduced to roleplaying games in 1975 with a Middle Earth game that appears to have departed heavily from Gygax's vision of how to play the game, although we don't get a ton of details about it.
Holsinger's articles are very clearly focused on creating a plausible campaign setting that enable "scenarios" to be run that feature large-scale miniature combats, with dungeon-delving as at best a prelude to that. He actually recommends in one issue that any new player who wants to join a campaign has to provide a prewritten scenario of this sort for the DM's use (and he seems to believe PCs will continue to do this as the campaign goes on as well). Holsinger and a number of other contributors to the Judges' Guild Journal's emphasis on realism is explicitly against Gygax's own statements on the matter about D&D.
Around this time the Judges Guild is also experimenting with modules - the first D&D modules available to the public so far as I can tell (Tomb of Horrors existed since 1975 but TSR hadn't published it by 1977). Holmes will launch with the Tower of Zenopus in 1977 as well, tho' it also provides little insight into what one is supposed to do with the game and rules beyond this example.
Anyhow, to cut this very long post short, it seems like until late 1977 there's definitely no single culture of roleplaying, partially aided by the fact that D&D is extremely vague about what you're supposed to be doing / trying to do when you play it. Judges Guild and the Strategic Review provide very different visions of the game - I'm sure if I dug through Alarums and Excursions I'd find even more variation. These aren't just minor stylistic variations either - Holsinger's elaboration of how a campaign runs in the JGJ is radically different that Gygax's, and Gygax is frequently complaining that people are playing D&D so incorrectly that it's no longer D&D.
It seems like from sometime in 1978-1981 there's an attempt to create a shared understanding of the game, expressed through the gradual publication of AD&D and of adventure modules by TSR and the Judges Guild (and the rollout of TSR modules at GenCon and other conventions to normalise them as models of play in the larger community), and to a lesser extent through artcicles in the Dragon and Judges Guild Journal. It's unclear to me how successful this effort is.
, with both having key texts come out in 1981 (the AD&D 1e DMG, and Call of Cthulhu, respectively), tho' I think Gygax beats Hickman's start by about two years since he must've commissioned Holmes Basic in 1976 for it to come out summer '77.