[electroking]

Sorry I can't help you with your questions regarding the Zenith, I trust
someone will come up with the info you need. I just want to make a
small comment about that 1950s Magnavox unit with stereo capability.
FM stereo broadcasting did not begin in the USA until 1961, but it was
understood for several years prior to that date that the stereo info would
be somehow transmitted in wideband FM, so manufacturers began to add
'multiplex output' jacks at the output of the FM demodulator, ahead of any
low-pass filters. If the set you are remembering was truly from the fifties,
it would use such a setup, and an outboard stereo decoder would be needed.
The circuitry would then involve a pair of audio jacks to return the output
of this decoder to the audio amplifiers in the console, and the switch position
you are mentioning would correspond to this mode of operation.

Some aftermarket stereo decoders were sold, but I understand their performance was nothing to write home about...

Also, a number of tuners and hi-fi consoles were built with the capability
of tuning in both an AM and an FM transmitter at the same time, for
some experimental stereo broadcasts, but these do not seem to have ever
been very popular. Actually, I know of at least one model of tuner that
had both this 'AM/FM stereo capability' (not to be confused with later
AM stereo systems!) and the MULTIPLEX jack. It was a Grommes unit
from circa 1960, but I would have to look up the model number. Regards.

(P. S.: Grommes 103GT)