msycamore linked to a YouTube clip of the original 1954 fanfare.

The one I posted is not a mystery - it was a new recording done in about 1981. The John Williams recording for ESB was never used outside of the Star Wars sequels/prequels. The version I linked to is the extended fanfare used on regular Fox releases in the 80s and up until 1994. Fox recorded a long version and a short version without the CinemaScope section (to replace the circa 1936 mono recording they were still using on most films).

I hate the 1981 recording of the long fanfare - the orchestra is weak and anemic compared to the 1954 version.

The original 1954 fanfare was on my old VHS, which was the 1985 remix. It was also on all the 1993-95 THX releases.