I may have missed the answer to this item somewhere in this very long thread, but if not, here's one item of info:
The MPX-70 was an outboard multiplex unit that attached to a regular FM mono tuner to allow it to decode stereo. My father had one of the first Fisher consoles with the multiplex option added, and we waited for KPEN in Los Altos to begin broadcasting in stereo, which finally occurred in July of 1961. Thus, my guess is that the MPX-70 was available in 1960 through maybe 1962. By 1963 the multiplex units were built in to the tuner chassis. And that's everything I know about it, because all I have left of that unit is two little tiny surround speakers, as the console vanished into Pacific Stereo as a trade-in some time in the early 70s.