Excuse me, but I don't see how this could count as such:
With extensive research, and while having much fun and bringing back fond, past memories while doing it. I managed to find one tiny paragraph, explaining some of what I was trying to properly remember.
Summary:
It DOES really need to be the ORIGINAL 4000 series, i.e. No A or B types. The 4000UB type isn't mentioned for that year, as far as I can tell, so it may not have been around then, or I didn't look hard enough.
The explanation, is because the original 4000 series, had just a single transistor complementary pair. Which, hence allowed reliable/stable analogue/linear circuits to be constructed.
Coincidentally, the original 4000 series, did not have the ESD protection, which was what I was somewhat remembering, in relation to this issue.
It seems to be saying that later types (4000A and if available in the applicable type then, 4000B), are too unstable, to use in (at least the circuit it gives), due to the extra transistors. Which cause instability.
There were other information articles/projects/examples, explaining about it. But they would take considerably longer to find.
Please refer to bottom of page 26, in the following link.
Source:
https://www.americanradiohistory.com/Archive-Electronics-Today/ETI-Circuits-No-2-1978.pdfI don't really definitively know if the (presumably) later 4000UB series, would be suitable, in those circuits. So you could be right (or not).
Does it really matter, since a modern, low cost, much better in most measurable ways, op-amp, would probably make a much better solution.
The original 4000 series, are probably getting increasingly harder to get, these days. I seem to remember, them being very difficult to get, even forty years ago (at least in the UK), as they were rapidly replaced by the 4000A and 4000B devices.