As msycamore said, the most obvious addition is the screeching sound when the first Y-wing is destroyed by Vader in the trench. This is not heard in the stereo or the '85, but it is a part of the 70mm version. I used to think it was something added for the special edition, since I had never heard it before; then when the GOUT came out and I learned about the existence of the '93 mix I believed it to have been first added to that version. But it's plainly there in the theatre recording, which really surprised me when I first heard it.
Interestingly, in the 70mm this sound is panned left, but in the '97 SE, mostly derived from the earlier generation four-track master which assuredly did not contain this sound effect, it seems to be on the right instead, since they had to add it separately and apparently chose to integrate it in a different way.
There is another addition when the TIE fighters first appear in the Death Star battle: the first X-wing to be destroyed has a different explosion sound than in the stereo mix. Actually, when I say 'different' what I really mean is 'enhanced', because I think the original explosion is still there, but with a loud additional element layered on top of it. There is also a short bit of the same screeching sound as the Y-wing example, too. The '97 mix does not contain these additions and the explosion sounds the same as it does in the stereo mix; presumably they either couldn't find those elements or chose not to include them (or maybe they just forgot).
The third example is the explosion of Vader's wingman when Han arrives to save the day. To me this really sounds like a newly created element, not just a different integration of the same sound effect as the stereo. Again, in the '97 mix this specific explosion is absent and was remade completely differently.
Few in number though they are, these changes are enough to show that the 70mm version was not identical to the stereo mix, though they are very close. So it is possible that the two surround effects I mention could also have been added along with them, after the stereo had already been finished. The '97 mix does have the echo in the canyon, but not the hyperspace effect. Some of the '93 additions made it into the SE while others are not heard in any other version, so it's just impossible to say with any real certainty. No two mixes have ever been entirely alike in terms of what is included or isn't, and with the surrounds absent in the recording it can't be traced quite as accurately as I'd like. Oh well.