This patent looks like a simple way to make a quad panner which could be used to produce the quad panning gimmick on some quad albums if you happen to be mixing down multiple tracks to 4 channels. Or you could use it for quad balance.
US 3811047 BALANCE CONTROL
quad optical
http://www.google.com/patents?id=9tEtAAAAEBAJ&printsec=abstract#v=onepage&q&f=false
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US 3911231 Four-channel stereophonic reproducing system
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US 3936619 Four-channel stereophonic reproducing system for reproducing discrete CD-4 four-channel stereo disc
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US 3985978 Method and apparatus for control of FM beat distortion
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US 4096079 Protective lubricating compositions for recordings
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US 3811047 BALANCE CONTROL
quad optical
http://www.google.com/patents?id=9tEtAAAAEBAJ&printsec=abstract#v=onepage&q&f=false
More patents of possible additional interest:
US 3911231 Four-channel stereophonic reproducing system
limiter 10
US 3936619 Four-channel stereophonic reproducing system for reproducing discrete CD-4 four-channel stereo disc
limiter 10
US 3985978 Method and apparatus for control of FM beat distortion
US 4198609 Phase tracked loop frequency demodulator
US 3871664 PHONOGRAPH STYLUS
US 4096079 Protective lubricating compositions for recordings
US 4096079 Protective lubricating compositions for recordings
US 4125668
Lubricants comprising dialkanolamine derivatives
US 4203856 Lubricants comprising fluoroalkyl esters
US 4232072 Protective compositions for recording
