Those ribbon cables are most likely used for the front panel meter, That meter measures current at the output stage so it appears each wire in the ribbon picks up the voltage from a transistor (probably across the emitter resistor) to do the conversion. Since there are a lot of transistors, it is much neater (and cheaper) to run circuit traces to the transistors and then gather them at a connector for a ribbon cable. Or else there would be a bird's nest of conventional wires if they were soldered point to point..
