not sure how they got the reverb in one speaker.
Some of the early "guitar amps with reverb" did not mix the with-reverb signal with the Dry signal, but instead sent it to its own power-amp & speaker.
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Sano -
Ampeg - Hammond AO-35 and AO-44 reverb amplifiers
These are "foldback reverb" designs, where the
speaker-output of the main (dry) amp drives the reverb tank, which is then amplified with its own power section & separate speaker.
In the case of the Hammond units, you could "add reverb to any organ" by connecting wires from the existing organ-speaker to the new unit's input jacks, and connecting its output to a dedicated speaker.
Fender tells you more or less that this is what they're doing:
... a unique speaker configuration of a 10-inch Jensen P10R and a 15-inch Jensen C15N. These speakers are driven by independent 20- and 50-watt power sections, resulting in an enveloping 70-watt wash of stereo sound. Next, the Pano Verb’s true spring reverb splash can be routed to both speakers or just the 10-inch ...
So the 2x6V6 "20w power section" is driving the 10" speaker, and the 2x6L6 "50w power section" is driving the 15" speaker.
Reverb is always connected to the 6V6 power section, but can also be connected to the 6L6 power section for "both speaker" use.
So the Panoverb is different, in that the Reverb must come from a shared preamp-section, but then can be sent to one or both output sections.