"The 'Lost Cut' survives today as a silent black-and-white 35mm copy in 13 reels."
Other quotes of interest from this article:
"...before the film was even finished, before the special effects and the shooting of the last scenes on the list - there was an early prototype of the film."
"While shooting in England was drawing to a hurried close in 1976 (the Blockade Runner interior shots had yet to be completed), Jympson assembled for Lucas and Kurtz a work-in-progress from the available footage - a rough cut of the film that lacked all the special effects, some scenes, and all the finishing touches, but which nonetheless offered a vision of how the movie was coming together."
"This early cut of the film was not meant as a polished work for presentation, and it was not the Star Wars that Lucas was ready to show anyone."
This is definitely a different cut of the film than the later rough cut (presumably in color) shown to Roy Thomas of Marvel Comics in February 1977.
"The main deleted scenes in the Lost Cut are those we already know: Luke at work in the vaporator field spotting the space battle overhead (seen as a lens flare with sparkles to indicate laser fire); his interaction with Biggs and the others in Anchorhead, and Han Solo's confrontation with a human Jabba the Hutt, which was restored and enhanced for the Special Edition."
I believe the Treadwell/vaporator scene on Behind the Magic came from the Lost Cut as well. It's silent, and it cuts to a "Scene Missing" card, which is characteristic of the Lost Cut with all its missing effects shots and scenes.
--SKot