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If you want to try with one CV out and one CV in, I'd suggest that you start first with using the VCF jack on the back of the Polysix as your CV In. Since it already exists, and should work, it'll be a good place to confirm that you've got your CV Out mod working.
For your CV Out, you get the MG signal out, but since the MG signal is already routed around to many spots (VCA, VCF, VCO), you should probably try to get one of the other CV signals. Maybe you should try for the PWM signal.
The PWM signal is generated on KLM-367. It's on the far right, a little more than half-way down. Look for the text "PW/PWM". If you tack-soldered a wire here, you would have your first CV signal. It would be unbuffered, the wire would easily break off with any handling of the synth, but it would be your first CV signal.
The main problem (beyond the lack of buffering and the lack of physical robustness) is that I don't really know what the signal looks like, so it might not be a very useful CV. Ideally, you'd like that signal to span 0-5V or -5 to +5V. I don't know what it does. You'll have to measure it...
Notice there is a test point marked in the circuit. You should get your multi-meter (aka "DMM") and measure the voltage at the test point on the circuit or at Pin 1 of IC15. Measure the voltage relative to ground, which you can pick up anywhere on the board with a ground symbol. Set your synth for PM. Turn the PW knob all the way low. What's the voltage?. Turn the PW knob all the way high. What's the voltage? That's the span of your signal. Is it 0-5V, or something else.
Now switch your synth to PWM. If you have an O-scope, set it to DC-coupled and use it to watch the CV waveform generated by the synth at this location. Turn the PWM amplitude all the way up. Does it span the same range as when in PW mode? Is it 0-5, or something else?
If the waveform looks useful (again, either 0-5V or -5 to +5V) you could experiment by temporarily jumping it over to the VCF in jack on the backpanel of the synth (or jumping over to the equivalent location on the circuit boards...which looks like KLM-366, down at the bottom of the schematic "VCF fcM In", jump to the correct side of R51.
Good luck!
Chip