I visited Abbey Road and went into some of their 'reverb rooms'. The plate reverb was just a big room with a couple of large flat metal plates down the middle. . . the signal went into a speaker in the room, then another microphone in the room would pick the reverberated sound up. . . you couldn't mix it, there was only one channel. Reverb or no reverb.
The above pic must be of a more modern desk than the ones I was looking at.