What do you mean with "late tone circuit production change by adding in a bypass trace around C6 and R4" ? That they made a mistake without knowing it ? Maybe C6 and R4 and only useful while rolling back the pot ! The moment you start rolling back, they enter the path. Probably they create a different shapping to the frequency when the treble is starting to be cut. Maybe a good mod would be changing R4, so that the effect of this knob while cutting treble would be less dramatic and still, the frequency cut point would be mantained.