msycamore said:

From the 2004 DVD audio commentary Ben Burtt says:

...the first mix done was a two-channel stereo Dolby mix wich hadn't been done before, it was a new process. That mix was done first because we were actually making prints and testing them out, and getting theaters to tune up their equipments required that mix was done first. After that was done, then a six-track stereo mix was done for the 70mm release of the film and of course going back and mixing the film a second time, one make changes, you take things a little further you've learned from the first mix of the movie and you now go and make all your changes in the next mix wich was the six-track...

I think the above quote along with the minor additions and differences in the six-track mix which also appears in the mono mix confirms that the 35mm Dolby Stereo mix was done first, but the 70mm six-track which was derived from the same source was first out the door... It makes no sense to remove certain sound effects in the 35mm Dolby Stereo mix then add them again for the mono if it was the second mix.

I think Burtt is referring to the different decisions made when mixing the "stems" together to create the final mix, rather than adding or removing sound effects.

I still believe what I said here, which is that the Dolby Stereo and 70mm 6-track mixes are identical in terms of content, the only difference being how the stems were combined to make the final mix.

The mono mix was a complete new mix created from scratch, and does have additional content. Some of these additions were also carried over to the '93 mix.