There is indeed an identical part — it seems that the balance control for amplifiers often has a “dual ganged” potentiometer

Linear behaviour is something that I had managed to conclude also, so it’s reassuring that you confirm this.

Do you know what the “H” in 200KBH means? That’s one of my big remaining puzzles. I agree that it seems likely that we’ll be best served by replacing it with something that is able to fulfill the same function due to not being able to find the specific part, but I’m not sure whether this could be something important.