This is the Plate Reverb from the upstairs at Smart Studios, Madison, WI - I was told it is an Ecoplate. The driver matches pictures of what was used on the ecoplates and the sheet is stainless steel. The small monitor speaker that hangs in front of the plate is used to drive the plate most of the time (although the mounted driver also works great). This speaker also came from Smart Studios and is how it was used there.
The difference between the two driving methods is that the mounted driver has more bass response, while the speaker has rolled off lows. Four transducers pick up the reverberated signal, which are amplified and mixed to a stereo signal and returned to the control room.