Here are two threads which have trailer lists/descriptions:
http://originaltrilogy.com/forum/topic.cfm/lt-The-Star-Wars-Trailers-Project-gt/topic/5649/
The second one has a 'File Name' category, so if you think the one you are looking for matches then it can be supplied.
I didn't know the differences in who made the trailers. Why would both companies make trailers at the same time? The description of the trailer you are looking for matches the 78 re-release until you mention the lack of Williams soundtrack.
There are a bunch of people familiar with the trailers, they'll hopefully chime in.