Very interesting!
This is what I am re-constructing out of this:
-stereo 2-track master made for initial batch of prints for premier day
-six-track mix made for 70mm screenings
-stereo 4-track master made (which replaces the initial 2-track master?) for subsequent stereo prints. This may have been an adaptation of the six-track mix made for 70mm, simply removing the LFE.
Alternatively, the last two points may be reversed. Maybe the 4-track stereo was made immediately after the premier 2-track, and then they added LFE on it for the six-track. Or maybe each of them is its own mix, who knows.
The mono mix definitely was the last mix done, as it has all the extra ADR and sound effects, and had the most amount of time to work on it because it was considered the most important. I suspect that Burtt's recollection about the tractor beam line not being in mono is simply a mistake on his part.
Mysterious uncoverings either way though.