From memory, the FCC system for stereo broadcasting had a 38 kHz carrier which is suppressed in transmission. To regenerate this carrier, there is a 19kHz pilot tone.
The two signals A and B were added to give the baseband signal. The two signals A and B were also subtracted to give A-B. The A-B signal was used to amplitude modulate the 38kHz subcarrier. The carrier was not transmitted. To regenerate the carrier, a pilot tone at 19 kHz was transmitted. At the receiver, the 19 kHz tone was separated out from the recovered modulation and used to phase lock a 38 kHz signal to it. The A-B signal was demodulated and an adding circuit used to add the (A+B) to the (A-B) signal. ie. (A+B) + (A-B) = 2A. Similarly, the (A+B) - (A-B) = 2B . Thus the A and B signals were recovered.
In the circuit, IC1 is a 'Quad Bilateral Switch'. Two of the switch units are used and the other two are unused. Pin 1 is the input of the 'left' and its 'output' is pin 2. For the other switch, pin 4 is the input and pin 3 is the output.
IC2 is a flipflop which appears to switch 'left' and 'right' alternately.
Because of the way IC 1 operates, it is difficult to see how the A+B and A-B signals are generated. I have to say that it is unlikely that this circuit generates an FCC stereo signal. So, what does it do?
It appears to sample two audio signals at a fast rate and to simply combine them. To recover the two signals it is necessary to switch them at the same rate that they were generated. There is no information how this is done. One would have to have a synchronous switch.
The components L and C5 constitute a parallel resonant circuit. For the values given, C5 = 10nF and L=6.8 milli henry, the resonant frequency is about 19 kHz.
IC4 appears to function as a unity gain amp with frequency selective drive to each input. The inverting input has a rising inut current with frequency; the in-phase input has a falling input current with frequency.
Bit of a mystery.