HH speaks the truth. As usual. 😉
The 5.1 was created for the final CLV MGM reissue, which like the THX THUNDERBALL merely condensed the big CAV boxset and removed the extras whilst adding a ac3 5.1 rendering of the previous matrix PCM audio. I don’t have this specific disc, but it should be identical and should be the source of the 1998 MGM DVD and it’s 1999 WB identical port. The video is the same master anyway.
The jump from 6 channel 70mm to 5.1 is indeed done as described above from what I understand, and should be how 2001 was handled. When WB got around to redoing all their Kubrick titles for the remastered DVDs they did their own transfers and junked all the previous ones that they had rushed out earlier. For 2001 they struck new 70mm prints for reissues and did their own restoration work, which apparently involved going back to the audio source tapes and creating a new modernized 5.1 mix without the original sound panning. This is why it was lost and has remained gone ever since. This is also where they lost the color timing and merely corrected their negative scan.
Comparing the modern WB 5.1 to the original shows the original feels not only more accurate to 1968, but also has more impact–despite this being comparing to the 6 track audio matrixed through DPL. The Criterion has some inherent damage so the MGM PCM is likely a better candidate since they were able to clean the audio before processing.
I wish I knew the generation of scope 35mm print I saw a few years ago. It was near perfect, had the original color intact with no fading, and was somehow the original mix in stereo and not a reissued/new WB print. I’m guessing it was pre-WB’s work and perhaps a late 80’s-90’s print in Dolby SR.