I tracked down several vintage HBO recordings in the past decade. It was a fold down of the stereo mix.

And the mono mix played theatrically. There were not a lot of theaters equipped for Dolby Stereo in 1977, and compatibility with mono gear was a technical issue not solved until much later. Nobody knew if Dolby would catch on, (other formats such as Sound 360 and Megasound have faded into obscurity) and Ben Burtt himself has said the mono mix was the one they took the most time on thinking it would be the definitive mix down the line.

The complete film was available on Super 8 in the UK and Europe in stereo.