I know that for musicals at MGM, in the 1940s, they often put microphones in 2 areas of the orchestra and recorded from 2 perspectives. These “stems” were combined for the mono soundtrack.I wonder where they found stereo tracks for a mono film.
If the stems still exist today, they can be combined for stereo. That is why late 40s musicals like “The Harvey Girls” and “Meet Me In St. Louis” have some numbers in Stereo on CD and DVD/Blu ray, depending on what stems are still in existence.
Perhaps this is what happened with the film you are discussing.