I have a thing for hand drawn traces.
This is on a Shure M267 mixer. I’ve seen at least three different variations on this same mixer. The one I’ve got at home contains only one transformer, at the output. This one contains five.
“Vactrol” style lighted photocell in optical limiter circuit. The audio envelope is detected, amplified, passed through a slow filter and applied as a bias voltage to adjust the light brightness. The photocell half of the device is used as a variable attenuator to back the volume down as needed to prevent clipping.
These devices are also commonly used in DJ mixers.