ChainsawAsh said:
The only difference I know of between the 77 and 85 stereo is, as msycamore said, 3PO's tractor beam line from the mono mix, which isn't in any other version until 1997. I feel like there are a couple other differences, but I don't know for sure.
The '85 remix also had more stereo separation (i.e. a wider "soundstage") than the original '77 mix. You can hear this in the jawa voices when R2 is captured.