Okay. So I just checked all the aux and effect send pots; the monitor volume pots, and the effect master volume pots and they're all plain on top (no engraved letter). I assume that means they're all A taper and the B engraved into the top of the shaft on my aux master pot is a B taper. I checked the effect master pot as well. Also plain.
So. It would seem that, unless it was very professionally replaced between its leaving the factory and me acquiring it second hand, that this unit has always had a B pot for its aux buss master control. Assuming the engraved B does indeed stand for B taper (and why wouldn't it?)