I came across this article where they discuss restoring this title.
They also talk about correcting the Cinemascope Mumps.
[color=rgb(121,121,121);font-family:Arial, Verdana, sans-serif;]Using original on-set production and publicity photos for accurate sizing reference, the restoration team removed much of this distortion by digitally squeezing in the image .5%. Since the original camera negative of 20,000 Leagues was actually shot perf to perf in 2.66:1, this new restoration has gained 1.6mm of image on the left side and .25mm on the right side of the frame-all of which had heretofore been concealed by the mag track in original prints. This “mumps” correction enabled us to faithfully reconstruct the original 2.55:1 image as seen in first-run theaters during the 1954 and early 1955 theatrical engagements.[/color]
If I'm reading this correctly, they're taking the entire 2.66 negative, and squeezing it to 2.55, and then say it's how people saw it in 1954.
People in 1954 saw the movie with the mumps, I don't understand why studios feel they have to alter the film for today's audiences.