These spring reverb units were manufactured by Gibbs, who also made them for Hammond organs. The units used a spring to create the echo effect, similar on Fender, and other amplifiers. They were a popular 1960's add-on.
I know on '66 C-bodies the switch was a separate knob mounted underneath the dash with a label indicating "speaker" which you pulled to turn on, and turned to adjust the level of echo.
The unit was mounted either underneath the package tray, or underneath the rear seats, I think the latter for '67 cars. I didn't know mine had one until I read how the speaker knob worked, and tried it. Adds great sound in a full sized interior. Got one for my station wagon, but haven't installed it yet......
I know for '67 (and also on '68 presumably) the same switch used for the rear speaker is pulled out to adjust the reverb if so equipped. You need the rear speaker to have the reverb.
They're sensitive to road bumps, so only turn it on for highway use.