I recently acquired a mid (July) 1960 Super, 6G4 amp (5 pre amp tubes). I was comparing the board to a a early "center volume" circuit and it is identical except for two 220K resistors in the trem circuit that are "missing" relative to the other amp I compared to and the schematic. At first I thought it was a mod but then I found another one (center volume knob as well) that had the same set up as mine. Also, there are no eyelets so it appears deliberate. Any idea what that might be and / or what difference that would make. Obviously they were experimenting a lot with the harmonic tremolo in this time period. Any thoughts?