@Irvrobinson ,
The center channel on those Mercury SACDs is not a mixed-mono channel.
Part of text from a booklet accompanying those SCADs:
The aim of the engineering team was to capture as accurately and completely as possible the true sound of the original tapes and 35 mm film masters for some of the discs. The multi-channel capability of SACD allows the listener to experience the recordings in their original three-channel format.
For the transfers to the SACD format, Saki Magnetics three-channel heads (specifically built for that disc series) were mounted on a Studer A80R ½ inch or Studer A820 1" recorder. Both machines were substantially modified to optimize tape reproduction and the analogue-to-digital conversion was done exclusively via DSD using dCS equipment.
Throughout the entire process, repeated comparisons were made both to a playback of the original masters on an Ampex 300 machine belonging to Wilma Cozart Fine and to the original CD transfers which she herself prepared.
Those late 1950's and early 1960's recordings were all originally made using a three channel recording process for the left, center and right channels. The same technique was also used by RCA for some of their 3-channel Living Stereo SACDs.