[Ron Ramirez]

1. What you are seeing is, most likely, BC - FM - PH. Where BC = Broadcast, short for "standard broadcast" - what we now call simply AM.

2. See above, plus I really doubt it says PM - may be difficult to read, but I really believe it says PH (for Phono or Phonograph).

3. Not to be a smart aleck, but it functioned as a tone control. In other words, it would make the brights appear to be brighter or the low tones seem to be lower, depending upon its setting. It didn't actually boost anything - it made the high notes seem higher by cutting the bass, and made the low notes seem lower by cutting treble.