While information about the '85 mix is hard to come by, certain qualities of its sound can lend themselves to informed speculation about its origins.
1) The '85 mix has full stereo imaging compared to the '77, which has been panned inwards.
2) The '85 mix shows somewhat improved fidelity compared to the '77, most noticeably in its lesser amount of tape hiss.
3) The '85 mix lacks the pronounced music reverb heard in the '77 during the quad laser battle and the end credits.
4) The '85 mix has been dynamically compressed to a greater degree than the '77.
5) With the exception of C3PO's tractor beam line, the '85 is identical in content to the '77. Nonetheless, certain sound effects occasionally 'feel' slightly different, perhaps due to the use of different compression and/or EQ during mastering.
Now let us consider a few other facts:
1) The 70mm version had a few additional sound effects not heard in the 35mm stereo mix, which also do not appear in the '85 version.
2) The '97 SE mix, which is said to have been derived mainly from an original 4-track master, does not contain these 70mm additions either.
3) The stereo imaging of the '85 mix seems to line up exactly with what is heard in the 70mm and '97 mixes (apart from the changes, obviously).
4) The 70mm and '97 versions do not contain the additional music reverb.
What may we deduce from this evidence?
1) Despite their overall similarity, the '85 mix could not possibly have been derived from the original 35mm stereo. The missing reverb alone would tell us this much; and the imaging discrepancy lends it further credence, for while there are ways of widening a stereo field, it is unlikely that artificial processing after the fact would yield a result that aligns so closely with sources that had not been narrowed to begin with.
2) The '85 mix must therefore have been derived from an earlier generation source that did not contain the reverb or any added sound effects, and had the full panning width.
3) The similarity of the '85 and '77 stereo versions, in spite of the differences they do display, is such that they must have a common ancestor in their background.
When we add up the facts, it seems to me that the most logical conclusion would be to say that the 1985 stereo mix is derived from the original 4-track master. It is a fresh downmix of an early, discrete channel source, with the tractor beam line added on top and the dynamics compressed down to a level that was deemed acceptable for home video release. This explanation accounts both for the similarities and differences between versions and makes the most sense given the audible properties of each, as well as fitting with the information available to us about these matters.