Stereo speaker input puzzler - Zenith C845

[Jeffhs]

My Zenith C845 radio has a terminal strip on the rear of the chassis, near the phono input and radio/phono selector, marked "Stereo Sp'k's Input". I tried hooking up a speaker to these terminals but got nothing. (The radio's internal speakers work as they should.) However, I am wondering what the function of this "stereo" input would be on a radio that obviously is not designed to receive stereo FM broadcasts in stereo, as there is no MPX decoder or second amplifier. Hmmm.

Thanks in advance for any info on this, as I am downright puzzled as to why a monophonic AM/FM radio such as the C845 would have an external speaker input. The only thing I can think of is that Zenith may have marketed a stereo retrofit kit for these radios that would have allowed them to receive stereo FM; however, I don't see any place on the tuning scales or on the cabinet front where a stereo indicator would have been mounted, and the band switch does not have a "stereo FM" position.