GreyLord
Legend
Just as prevalent? How about much more prevalent?
I can't speak for everyone, but my experience playing 1e (of which I have far too much) is that tables picked and choose what rules they were using, although there was a "gestalt" of core 1e rules that the majority of tables followed, and it was (in certain, nerdier corners at least) a mark of esteem when you could recite weird Gygaxian rules buried in the PHB or DMG that people weren't generally aware of- not that the table would follow the rule, just that you knew it.
My experiences jive with yours.
In my case, I was the youngest person in my original group of players and looking at where we all ended up, the most driven and opinionated. So as we embarked on our nerdy corner the only way I could avoid being taken advantage of by the DM (and later on, players) was to know the PHB and DMG well enough to be "that guy" and back it up on the fly.
Actually super helpful for later life, because it wired me for detail driven professions and public speaking. Of course, doing that put me behind the 8 ball on social skills. While I'm now comfortably in the "normal" range; I had to learn etiquette programmatically, and not experientially for it to stick.
So I guess my idiot savant skill is rules frameworks of any kind. (D&D, Cyber GRC, Coding languages, Academic programs etc.)
This is why most people could go from one table to the other and easily transition in AD&D. The Core assumptions at the table were the same, but there were many miniscule rules from the DMG or elsewhere that were commonly dropped.
On reflection of the time, I think much of that originally was in regards to the original D&D rules. The core booklets along with Greyhawk and Blackmoor were common ground that were used among the old wargamers that came in with OD&D. A LOT of the stuff from Strategic review or other sources were very hit and miss. Thus, the common ground that many would come from would be the three core booklets and the supplements (and normally only the first one or two).
These lacked a LOT of the additional rules that got tossed into AD&D.
In addition, because AD&D didn't get released all at once, people just played with the books that came out using the OD&D rules. This made it so that OD&D really was the prevalent version of AD&D...but with more classes and more defined monsters.
Later players didn't have this privilege. My guess, though, is that may of them started with the Red Box and Red Book of Basic (either B/X or BECMI). After learning the basics of the game, that's what they stuck with. They moved on from Basic or Basic and Expert right on into AD&D. The games are similar enough that you can intermix and so they played AD&D with the rules which were the same between Basic and AD&D and skipped most of the rest of the rules which added complexity.
Thus it was the things that were the very core ideas between OD&D, B/X, BECMI, and AD&D that formed the central core of the all those games and made it easy for people to go from table to table and place to place as everyone was using those core ideas.
Even 2e used those core ideas for the most part and thus that transition was pretty easy. (plus 2e had a grandfather clause that allowed a group to take any 1e thing and use it in 2e if they so desired...though I think that was more spelled out via other media and NOT found in the core 2e books).