Don,
The seller included an amazing ~300 photos and some sorting through will find that it's labeled "SPEAKER".
As far as I'm aware, such a knob was not included when you got the rear speaker, yet it appears original. Wonder if it was something on the 56's only.
The RPO chart shows that the 3-way radio comes with the electric antenna and rear speaker: "Radio, Three-Way Tuning with
Electric Antenna and Rear
Compartment Speaker*"
I have never seen another one with that knob though, and even the normal radios did not have a "Speaker" knob; they had Tone and Tuning knobs, so it wasn't just pulled off of something. Maybe someone else here will know.
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