I wonder if you’re thinking of a trailer for the movie? Sometimes they put things in trailers but not the movie. For example when a Tie Fighter rises up to confront Jyn and has her in its sights as she’s on the parapet outside of a tower on Scarif.
Or maybe you’re thinking of the fact that it takes Luke a (seemingly) long time to uncoil the rope?
I’m quite sure the answer to the first item is no. All of the (known) trailers are available and none contain this. I certainly agree that trailers often contain additional footage or alternate takes. I’m also quite sure I never saw a trailer for Star Wars before seeing it the first time. I had never been to that theater or any other where Star Wars was first shown, and had only ever been to the movies 2 or 3 times before that. I heard about it by word of mouth, and was invited to go with neighbors to see it.
Again, watch the specific shots I pointed out at 1:49 and 1:55 very carefully. They give the clues to what should be between them. Then listen to the music very carefully. I believe the music very clearly tells the story as well. I believe the changes (inserts) were frame for frame replacements (141 replacing 141), so there was no change to the music or timing necessary.
I have carefully listened and compared the audio recording of the 70mm presentation at New Jersey’s Triplex Paramus during its initial run (link below). I’m quite sure that the frames with Luke’s missed attempt were not present here. The audio well matches the blaster shots in the inserts that replaced it, with no sound that would match a missed grappling attempt (or reeling the line back into the belt). This is why I think if anything, the alteration was made before the eight first-run 70mm prints were struck, leaving only the thirty-five first-run 35mm prints as possibly containing the full missed attempt shot.
http://www.wideanglecloseup.com/starwarsaudio.html (The chasm scene is near the end of Part2.)
Again, I certainly consider it possible that the combination of the music for the scene combined with what is seen around these 5 7/8 seconds is so suggestive that it has lead many of us to believe that Luke missed, even though we never saw it that way. However, I think it is more likely that some of us actually saw it, and even more likely it was that way when Williams scored it.
Another possibility is that Lucas convinced Fox to send this 141 frame replacement to the theaters shortly after the premier, with instructions for the theaters to make the edits. That kind of alteration was certainly done with 2001: A Space Odyssey in 1968. However, I think it unlikely that Fox would have done this in 1977. This was a first run exclusive, not a roadshow presentation like 2001, and I doubt Fox would have been anxious to participate in that kind of alteration to what had just become their greatly unexpected super-hit. Lucas of course was tinkering at every opportunity (as evidenced by the changes in the monaural soundtrack for the general release he and Ben Burtt made a month later).