Another interesting tidbit:
In the early days of stereo, a session would often be recorded using two sets of mics, two boards, and two engineers - one for the mono and one for the stereo. As a result, there would literally be a separte mono and stereo recording, as opposed to a separate mono and stereo mix. The overall sound and presence of various instruments can vary radically between the mono and stereo recordings on these types of sessions.
Many of the early stereo Capitol sessions, for example, were done like this.
Speaking of Stan Kenton, "Kenton in Hi-Fi" was later released as "Kenton in Stereo," with a recording setup such as the one I described. The stereo version sounds very experimental in comparison, and has virtually no bass (the instrument, I mean). The mono is the one to have.