Interesting. With digital quality (whatever that is..) everywhere, no-one knows much about the analogue world, except perhaps for a few vinyl enthusiasts. I recently restored a Fisher receiver, and had to learn about how the FM stereo circuitry operated, to understand what was really going on in the alignment of the MPX decoder, and its correct operation. This being mid 60's equipment (of quality), the selector allowed for Mono, MPX permanently active and an automatic mode driven by a detector for the 19kHz pilot. Nicely implemented, and very effective in practice.