It was pretty common. RCA, GE, Dumont, and many others had that practice. It started to fade once transistor based devices starting pushing tube based stuff aside. The SAMs docs picked up the slack.
I have experience with several early RCA TV models, none had schematics inside. No idea about GE or Dumont, though.
It was actually quite common for early transistor radios to have schematics inside, but they were generally too small to be useful, even with a magnifier.
Bottom line...you need more than a schematic to fix more complex problems, to do alignment, and the like. It really makes no sense to include a schematic inside the cabinet, it just encourages Mr. Fixit types to fiddle around and cause more problems.
I know. I drove my parents nuts that way when I was a kid!