Did find this from Radio and Accessory Repair Information for Chrysler Imperials and Imperials
While that site isn't the easiest to navigate, there's a lot of good info, and not just for Imperials.
OPERATING INSTRUCTIONS FOR REAR SEAT RADIO SPEAKER EQUIPPED WITH REVERBERATOR
Your new automobile is equipped with a factory installed combination Rear Seat Speaker and Reverberator unit which produces outstanding sound reproduction.
How it Works: To simulate "music hall" acoustical effects, audio waves are delayed by an Electromechanical delay line and reamplified after 20 to 30 thousandths of a second. The final output is unique and of distinctive quality.
Reverberator Control: The Reverberator is controlled by a push-pull knob located near the radio. To turn the Reverberator "ON", pull the knob out. To turn the Reverberator "OFF", push the knob in. With the control knob in the "OFF" position, normal speaker performance is obtained without reverberation. Adjustment of the volume "mix" between the front and rear speaker is obtained by rotating the control knob. With this built-in fader control feature, the rear speaker control ring, located behind the right radio knob, is no longer used for mixing between speakers.
NOTE: The radio and the Reverberator are separately operated power units. When the radio is turned "OFF", the Reverberator should also be turned "OFF" by pushing the control knob in.
INSERT THESE INSTRUCTIONS IN YOUR OPERATING INSTRUCTION MANUAL
Part Number: 2497388 - - - - - - - - - - - - - - - - - -