Hello Ed, This is delving back a good many years! The AM and ACM modules all ran on 15-0-15 rails and these appeared on pins 13 and 15 of the edge connector, but there the memory fails. Usually audio ground was on pin 24… but don’t bank on that one. I’m afraid I don’t have any circuit details from those days, but the mic amps were conventional transformer input using mostly Sowter transformers. In the mixers we used quite simple EQ; variations on Baxendall HF and LF with sometimes a simple tuned mid.
Send me a picture of the modules and it might well jog more memories!