This is a good discussion and it should be pursued, perhaps without being conclusive, knowing that without external intervention (from the original manufacturer) such an end goal or consensus may not even be possible.
While period correct catalogue listings are always an excellent form of period correct documentation, they would only validate what the intended correct arrangement (in this case reference no to model) would be, not assembly anomalies. My point is that as a collector of a wide range of vintage watches (not claiming to be an expert on any), if I compare this situation/scenario with known, popular, well documented examples or references from Rolex, there are (acceptable) instances of the Submariner 5513 with casebacks that are stamped 5512 (they were essentially the same model family - one with 2 lines of dial text and another with 4). Even numerous datejusts (models such as the 16014) would carry a generic 16000 caseback all the way into the 80's. This was not unusual for big manufacturers and would not be out of place for the likes of UG in my view. If we looked at the Rolex catalogues for these references from that time period, they would obviously state what the correct reference numbering would be, but would in no way nullify the existence (and indeed, acceptance) of such anomalies.
While parts may be available, it also strikes me as odd why we don't find an equal number (or even any occurrences) of the reverse - where Evil Nina's are found with the Nina caseback? And where is the oversupply of these casebacks coming from - were the Evil Nina's produced (and subsequently damaged/lost/sold as parts) in larger proportion than Nina? Wouldn't someone trying to cobble together a reference to sell for a profit find it easier to pair the caseback to the right dial and sell that arrangement more convincingly with ease rather than to the wrong reference and have to justify how the arrangement came to be? I don't have the answers, but these are the questions in my mind....
The fact that more than one exists is exactly why this discussion is happening - to me its not about getting it wrong but about a pattern.
As we speak, I've found yet another 2568xxx in the far east (Singapore) on sale with the same MK3 configuration and a /01 caseback .
Link:
https://pushers.io/listings/universal-geneve-compax-nina-rindt-885103
Picture for reference: