Re: Stereo balance pot
The fourth pin could be some kind of centre tapping on the track which would short one half of it out maybe? The arrangement I've used myself for basic balance controls is the pot across L - R with an earthed wiper, but I'm pretty sure that some older amps (the Leak Stereo 30 for example) have one track for each channel which has no resistance past the halfway point as Roffe has pointed out, so that the volume in (say) the left channel won't increase any more between centered and fully left.
A standard dual gang pot would probably be better than nothing. Where does the flying lead go?
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