I spent some time this evening reversing what is going on inside that black box, or perhaps I should say what might have been going on if it was connected up. One thing I can say is that it is definitely not purely digital in nature.
I can see lots of pull down transistors, which are the transistors whose gates the various bits in the shift register are connected to. And there is definitely a pattern to those pull downs. What we have is this:
SR7 and SR0 are connected to two pull downs in parallel to each other (a bit like if they were NORed together, if there had been a pull up)
SR6 and SR1 are connected to two pull downs in parallel
SR5 and SR2 are connected to two pull downs in parallel
SR4 and SR3 are connected to two pull downs in parallel
In addition to the the above, we also have:
SR6 and SR1 are connected to two pull downs in series
SR5 and SR2 are connected to two pull downs in series
SR4 and SR3 are connected to two pull downs in series
Given that pattern, it seems strange that there is an absence of SR7 and SR0 connected to two pull downs in series. Instead where I'd expect to see that, we have the area of the die shot that has the two polysilicon lines that I mentioned previously were not connected to anything. I think that these would have originally been where the two pull downs in series for SR7 and SR0 would have been. Instead it seems that SR7 has been disconnected from the polysilicon line that it would have used, SR0 disconnected from the line it would have used, the diffusion link between the two polysilicon lines removed, and SR0 connected to the polysilicon line that SR7 would have been connected to, creating one pull down, which is then connected straight out to the output via a metal line.
There are also a number of transistors that I think are acting as resistors. A good indication of this is when the dimensions of the transistor gate are non-standard for a normal logic gate. When you see all kinds of large shaped transistor gates of varying shapes and sizes, then there's a good chance that they're acting as resistors (although sometimes they'll be a capacitor, so you need to look at whether the transistor gate is connected to the diffusion on one side of the transistor to confirm it is a resistor; it wouldn't make sense for a capacitor to have such a connection).
I think the original design might have had a reference voltage connected to the two pull down transistors in series for SR7 and SR0, then a resistor, then the two pull down transistors in series for SR6 and SR2, then a resistor, then the two pull downs in series for SR5 and SR2, then a resistor, then the two pull downs in series for SR4 and SR3, then a resistor, then the two pull downs in parallel for SR4 and SR3, then a resistor, then the two pull downs in
parallel for SR5 and SR2, then a resistor, then the two pull downs in parallel for SR6 and SR1, then a resistor, then the two pull downs in parallel for SR7 and SR0, and then that connects to the output. All those resistors are different shapes and sizes though. I can tell by looking at them that they'd have different resistances.
I don't know much about analog electronics, so not sure what such a circuit would have done. So perhaps it is good that they decided to disconnect it, because now we don't have to worry about it for the purposes of understanding how the 6561 sound works.
Would certainly be interesting to see a die shot of an original pre-VIC 20 6560 (as used in Attack UFO).