when staring down at the pot, there's nothing soldered to the left "peg" (sorry the name escapes me). the middle peg has the the blue wire from the obp-1 and a white wire from the bridge vol. knob. the right peg has a capacitor (473k/.22 micro-farad) soldered from the right peg to the top of the pot. there's a ground wire also soldered to the top of the pot.
Sounds like a passive tone circuit wired in ahead of the OBP1 (blue wire from the OBP1 is the input). Also often called a "vintage tone control" when used along with an active EQ. Since passive tone circuits act differently from active treble EQ, some like to have that additional "flavor" available. Doesn't really change anything when left wide open, so if you've got a hole for the knob, might as well have one.
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