As I said above, they worked on the 4-track master first.
That is the original mix.
Both the 35mm stereo and 70mm 6-track versions were then largely copied from it, with some changes in presentation made to each, mainly for technical reasons. The metaphor of twins is an apt one in this case, since they were completed within a very short time of each other. The only reason they weren't done simultaneously is that it wasn't physically possible to print more than one mix at a time with the equipment the mixers had available.
If you really want to hear the actual, true first mix of the film, you'd need to go to LFL and pull out the 4-track master tape and listen to that. This would be the only way to hear the 'most original' copy of their work, if overly specific definitions are to be used. It would sound very similar to the stereo mix except with discrete channels and an unrestricted dynamic range. (But no bass.) The stereo version is a compromise derived from this master, made in order to accommodate the limitations of a delivery format which could not contain the full mix in its entirety.
Rumor has it that some 35mm prints were actually made in 4-track magnetic format. This would yield a sound closer to the source, but it still wouldn't have been as powerful as the 70mm, owing to the slower speed of the film going through the projector (in analog sound, slower speed equals lesser fidelity), and the lack of dedicated bass tracks.
In the end, arguing over strict originality is silly, because no released version is completely identical to what was first put on tape. It becomes a question of which do you value more: the fact that one printmaster was finalized a day or two before the next, or that one of the mixes retains the superior sound quality and power of the source and is further optimized for that purpose? The fact is that both the 35mm stereo and 70mm 6-track versions were heard in theatres on the film's opening day of May 25, 1977. Both were sourced from the same master copy. Neither is entirely the same as the version they were derived from. Both mixes provide a great aural experience for the film, depending what kind of sound system you have and what its capabilities are. How much more 'original' could you possibly get?
(The mono mix doesn't really apply to this discussion, since it was remixed entirely from scratch and didn't appear in theatres until a month later. Sam Shaw, who is credited as the film's Supervising Sound Editor, didn't even work on that version. The multitude of changes make it the earliest example of revisionism in the film's long history of being altered. Nonetheless it was part of the theatrical run and the only mix heard by many people at the time, so it 'partially' counts.)
At any rate, the really remarkable thing about all this is that we've been able to preserve or approximate a great deal of the work that was done on the film back then, allowing it to be seen and heard at a quality level that was unthinkable even a few years ago. Recognizing that the goal of somehow achieving a perfect, 100% authentic most original ever version is an ephemeral one at best, given its history, being able to sit back and watch the movie without being frustrated by shortcomings in presentation is a pretty great feeling. Let's not lose sight of the fact that this is ultimately the reason why we're here in the first place. ;)