I seem to remember reading on imdb that it was 1.81:1, which would’ve roughly split the difference between the 2.39:1 aspect ratio AotC was actually finished at and the 1.43:1 of the Imax prints.
Interesting, I guess they were actually trying to accommodate the movie for IMAX screenings even if it meant losing image (ironic really). Unlike many after which were just the regular version on an IMAX screen, i guess that’s why we have Dolby today