Anybody have an idea what this is?

[orthophonic]

That Zenith dates from 1958/1959.
It is the stereo "companion" speaker amp.
It was common in the early days of stereo for the second channel (to make stereo) to be offered separately.
this allowed the buyer to purchase the main unit (with turntable/amp/speaker) and upgrade to stereo playback
by purchasing the matching companion amp/speaker when
he could afford the additional cost. It plugged onto a special jack on the main unit.
This was done with consoles and portables.
This "companion" speaker/amp can be used by itself by plugging a line level source into its input.