Y'know, I'm not sure exactly what was done, or if this is the way it came from the factory. Here's the unit itself:
Here's the tuner/pre setup, below the preamp are the two crossover adjust knobs :
There's a Zenith OEM decal on the back that has the tuner model number and a tube layout. It also seems that someone added a mono/stereo knob on the back with again what looks like a factory decal, but someone had to notch out the vent holes in the waferboard for it. I'd take more pics, but my camera's out of charge......
This is an interesting find indeed. Figuring what it is, alone, is worth the $25 I paid for it