The 4711 Mixer is a four channel mixer with pan controls for left and right outputs. This kit was $29.88 in the 1976 catalog.
4711 PAiA Mixer Manual
4711 PAiA Mixer Schematic
These are the bottom traces. The two resistors are bleed resistors for the output capacitors. The top bus wires connect to the 3.5mm jacks.
I ran ground wires to all the 3.5mm jacks. The power cable terminates in a 4 pin MTA connector. The ICs had already been upgrade to TL072.