Unknown potentiometer in schematics
Figure 1. A four-terminal or centre-tapped potentiometer - not to be confused with a pot with a fourth terminal to connect an earth wire to the case.

Four-terminal potentiometers were common at one stage on hi-fi volume controls to give automatic "loudness" control - see What&#39;s a potentiometer with four terminals in this schematic?. These were logrithmic law or "audio taper".







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Probable balance control circuit.

My guess is that it's a linear pot and that the centre-tap is used to provide a minimum load to each side.


With pot in centre position the signal fed into 'L' is \$ LEFT \frac {R3}{R1+R3} \$ and the signal into 'R' is \$ RIGHT \frac {R4}{R2+R4} \$.
If we move the pot wiper all the way to towards 'L' we end up with \$ L = LEFT \frac {R3}{R1+R3}  = LEFT \frac {0}{R1+0} = 0\$. The 'L' channel will be fully muted but 'R' will remain unchanged.
If there was no centre-tap the right channel would get loutder in this situation. This would normally be considered "a good thing" as the overall volume would remain the same and you are only changing the "balance" between left and right. In this application the designers may have reasons for not doing this.