Elmo seems to have such a fascinating early history.
Starting with my history of learning his early history, the first time I saw anything about becoming a major character was on one fan site around 1996, I can't remember what site, but it said that Elmo was a minor character until 1984, I think it also said that Kevin Clash started performing him there. I don't think I thought too much about an earlier performer. None of the (fan-compiled) performer guides listed anyone else for Elmo, but I figured out by then that some characters didn't have consistent performers (and assumed that for every recurring character not listed, though mainly for TMS characters I knew about but didn't know the names of - there were so many of those I didn't know the names of and, when it comes to performers and their characters on The Muppet Show, barely saw any names I didn't recognize).
A few years later, I saw the Sesame Street Beans in stores, was looking at them and noticed the tags had facts about the characters, including what years they first appeared. This interested me so I looked at the tags for debuts, and saw 1984 listed as Elmo's debut, confusing me because I'd previously read he became a major character then. Quite a few things didn't occur to me - 1. that non-fiction information, even officially written, could be wrong 2. that fan sites should not be considered the definitive source for these kinds of things 3. that when I saw contradictory information in non-fiction things that, while confused, I didn't question it too much (and often accepted the more wrong-sounding info over what sounds more right).
Sesame Street Unpaved was the first thing I saw that listed other performers for Elmo. I still knew he was a minor character before Clash took over, but didn't know to what extent he was minor. I also thought Brian Meehl and Richard Hunt were the only other people to perform Elmo. And it was then that I saw Elmo debuted in 1979. When I first heard another Elmo, in the A&E Biography that showed a clip from "Me, Claudias", I assumed that was Meehl, since I wasn't very familiar with Meehl's voices. I never thought it was Jerry Nelson (even after being informed it was Jerry for this and other early segments, I have trouble thinking of other Jerry Nelson characters with that voice), but I was definitely sure the announcer was wrong about it being Richard in that segment (since it doesn't sound like any Richard Hunt characters I know of).
And then a few years later, I read in Caroll Spinney's autobiography that Elmo was originally called Baby Monster and appeared in the early seasons, long before being brought out of storage and renamed Elmo. And a year or so later read that Caroll Spinney said he performed him in his "Baby Monster" phase, confuing me a bit because I thought Meehl and Hunt were the only pre-Clash performers, but I figured it was different if he was called Baby Monster. And after all these years it seems Spinney's memories on Baby Monster/Elmo were incorrect. And then it was around this time that I started hearing that in his early years he was a generic, unnamed (or at least implied to be unnamed) monster.
And now I'm remembering that, before reading Unpaved, I noted Elmo in a book I had that had the copyright date 1981 (I actually can't remember the title right now, I'm thinking "Circus of Opposites"), so that let me know that Elmo was indeed around before 1984. I pointed this out somewhere on the wiki in regards to the earliest he was named, before the wiki added another naming source: a season 12 press release that referred to him by name, which I first saw in 2009, and assumed that was when he was named (since it was referring so many new characters for that season).
Then the first few weeks of season 12 were leaked online and I started to realize that it seemed Jim Henson, Frank Oz, Jerry Nelson, and Richard Hunt were barely there for season 12 (perhaps more so than other TMS-era seasons) and, in addition to knowing that Caroll Spinney, Michael Earl, and Brian Meehl were the only puppeteers who performed on a full-time schedule on the show, noticed that pretty much all that seasons characters (including some whom I either didn't know the performer of or had seen incorrect performer info) were performed by either Meehl, Earl, Karen Prell, or Kevin Clash. So it got me wondering about Elmo's performer that season. I didn't think Meehl would have performed him in season 12, since he's said he only remembered performing him a few times, I assumed he started performing the character closer to leaving the show. Since he was noted like that in the press release, even if he only spoke in sounds, I didn't think he seemed like the kind of character who would be performed by whoever was available on set (and for season 12, it seems there weren't many performers to alternate with the character).
But then this recent info has surfaced. It's pretty obvious Caroll Spinney was wrong about the "Baby Monster" details, though it's unclear what he was thinking about (I suspected maybe he was thinking about Billy Monster, who does kinda have an Elmo-like look and is small enough to pass as a baby, then I learned that at least one season two script refers to Billy as "Baby Monster", though it's for a sketch where Fran Brill performed him; incidentally, I feel the voice Brill gave the character sounds close to Elmo's voice). And then we found that Elmo WAS named Elmo in a season 11 episode, that Brian Meehl did perform the character as early as season 11, and yet despite having a consistent performer and personality on the street, was performed by anybody (usually Jerry Nelson, in cases we know about) in inserts, sometimes called "AM Elmo" in scripts, and didn't communicate by mumbling or speaking in single words. I don't think any other character got this kind of treatment when they started out as actual characters.
Another thing of interest is that, when Brian Meehl was interviewed and talked in the voice he used for Elmo, I feel it sounds close to what Jerry Nelson did. When I started watching clips on YouTube, I noticed that some of the one-shot characters Meehl did sounded like the voice Elmo had in "Me, Claudias", which is pretty much the voice he spoke in.