The-long-and-the-short of it is that when the film was released on May 21st, 1980, it was a limited engagement that was shown only in 70mm prints. When the film went into general release with 35mm prints on the following June 18th, it was new a cut that included changes (some optical-effects fixes and dialogue replacements, among them) and 3 new special effects shots that increased that length of the film by 20 seconds.

Discussion of that limited-engagement cut can be found here.
http://originaltrilogy.com/topic/ESB-70mm-Soundtrack-1980-in-theatre-recording/id/12501/page/1
https://originaltrilogy.com/topic/Hopefully-the-last-70mm-vs-35mm-ESB-audio-differences-thread/id/14128/page/1
http://originaltrilogy.com/topic/70-mm-print-of-the-Empire-Strikes-Back-Differences/id/2321/page/1

Someone did a recreation of the ending of that limited-engagement cut on YouTube.
https://www.youtube.com/watch?v=cmbl8tLOfFE
The audio in that video is a vintage In-Theater recording of a screening from that alternate cut.

Details about that audio recording can be found here:
http://wideanglecloseup.com/starwarsaudio.html