Do you remember when an electrical schematic was pasted to the bottom of every electronics device?
or on the inside back of radios.
I still have a tube tester and a heath voltmeter that uses tubes believe it or not.
It wasn't actually that common, but Sears-Roebuck (remember them?) did that with a lot of their Silvertone products. They even included parts lists that people could order from.
What's important was that publishers like Sams (via their Photofact service) provided detailed information to repair people and technically-oriented owners at reasonable prices. They still do, but their stuff is quite pricey now and was occasionally inaccurate.