[old_tv_nut]

The values of the plate load resistors are too blurred to read, but the magnitude of the balance adjustment will depend on the ratio of those to the 100k balance control.

AC-wise, at one end of the control setting, R60 is shorted so total AC plate load is R58, and at the other end, 100k is in parallel with R60, so total plate load is R58 plus (R60 in parallel with 100k). The opposite effect occurs in the bottom section.

When balance is centered, total AC plate load is R58 + (R60 in parallel with 50k).

You should be able to hear the volume decreasing/increasing if you listen closely to one side as you turn the control.