I think you just sent audio to it and bought the output back, not that familiar with analogue audio terms but I guess it went in the FX send chain and came back in the FX return. No doubt this monster had it's own control panel, I can remember the old spring reverb units and I'd heard of studios with large pipes buried under them that were used a reverb chambers.
Actually thinking about it maybe what all the motors and gears in the unit did was to move the position of the transducers accross the plate, that'd make sense, you could alter the delay that way.
The thing I like about a lot of that old stuff is you could see how it worked or even show someone how it worked as so much of it was mechanical, try explaining to someone how a convolution kernel works, I'm doing fine if I can spell it right!
Bob.

Would someone hand me my walker...

Plate reverbs were cool. They were designed to mimick natural reverberation when no "room " was available. In operation, sound would vibrate this large sheet of metal. The amount of reverberation created would be determined by the amount (volume) of signal directed onto the metal and the location on the metal where the reverb was "picked up". Kinda like shaking a sheet of steel and making it sound like thunder.

Spring reverbs worked in a very similar fashion, but most lacked the finese of a true studio plate system. Did you ever hear a gutiar amp being dropped and thundering boom come out. That's the spring reverb getting a jolt. The sounds of these units weren't bad, just different. Usually, plate and spring reverb presets are included with reverb plugins.

Controls for these devices usually consisted of (1) level going in and (2) level coming out.

I have an old Fender spring reverb unit in storage. Used it back when I was "Born To Be WIld"!

That old Fender unit was great. Muting the strings almost sounds like gunshots. Great for songs like "Ghost Riders".

I had a Hammond organ with a spring reverb. When I rocked the organ back and forth on stage (which I did habitually) it sounded like thunder. I felt like my namsake.
Tor

I've used many a plate reverb unit in my career. The real high end units (i.e. EMT) also used motors to move the plates themselves in proximity to each other. Some of the units sported gold plate on the plates, and they could cost upwards of $20,000 (that's mid '70's to mid '80's dollars). Back in the 1960's & 1970's, BEFORE digital reverb was a technological practicality, broadcasters - in fact, ANYONE who wanted artificial reverb- was forced to use spring or plate reverb. Plates were larger & more expensive than springs. They usually tucked them aaway under stairwells or put them in storage rooms and ran i/o lines to them. Here's a tidbit: at the end of the 1970's, but especially in the early 1980''s, digital reverb became do-able, but at VERY high cost. In the early '80's, THE digital reverb unit of status was the AMS RMX-16. In it's basic configuration this thing cost $16,000, but that only got you a very limited amount of reverb time, due to the limited amound of RAM included with the base unit. Maxing it out witha bit of extra RAM drove the cost of the box up to $30,000.