Some industries used amplifiers to drive "vibrating tables" for certain processes. Perhaps these did something similar.
The old Bogen E series amps looked like those but had more knobs. Some old Stromberg Carlsons did too if you could find one with the cage.
Simplex also had something similar they used with their 35mm commercial projectors. I used to come across those in older installations around Hollywood and Beverly Hills. Many were torn out when I upgraded them to Dolby surround systems in the 80's.
I had similar things at the end of 21 but a little more recent (thyratron + timing relay tube inside) still used for a fishing net production line
We had a giant shaker table at Motorola for simulating the torture a mobile radio would get in the trunk of a cop car under "extreme" road conditions. The old system had a giant vacuum tube amp and a function generator that shook the table with sine waves of varying frequency and amplitude. Sometime in the late 80's it was replaced with a solid state system that used digitized "programs" with plain text names and variable "G" force settings. My favorite was a program called "Military Jeep" it could destroy nearly anything built back in the 80's at any power setting above 10.
Electronic carillons ("bells" for church towers) had very plain looking amps like that. All the tin is either standard parts or made in-house on the same tooling as commodity chassis suppliers.