In the case of... let's say "rare".... dials coming from respected collectors like the above Monaco, I take the position that such pieces should only be purchased by the absolute crux of collectors, those with inside connections to the heritage departments or museum curators of the brands they purchase. Such collectors are likely to be able to not only verify the watch from as close a source as is available (again like you say, they are not always the most reliable) but also tend to be in such financial positions as to be able to write off a bad purchase without it significantly affecting their lives.

For the average collector though, I think it's better to pay a premium for a "run of the mill" regular version of any vintage watch (i.e. commonly accepted to have been produced by the marque at the time), so that the likelihood of a "prototype" actually being fake is lessened.

 

I remember this Monaco was offered to me two years ago, is it the same watch?
Scary stuff!

 

Just found this dial for sale...uh oh.

 

And found this on the On the Dash site: The Diarex, under the Poor Man Monacos section, interesting I guess.

 

And another one, the Eugen Siegele "Monaco" 1972 ("Eusi") :


Considering there are that many "variants" around of this particular dial setup, I would skip the whole thing as a collector, the chance somebody fiddled with the dial (had it reprinted with the Monaco and Heuer logo) is way too high for me + no reference from Heuer in the official literature anywhere about this dial color configuration.
So how shall we call these watches...hommages, as in: looks like it?

 

You bring up a good point, which is that at that age there's no hard and fast rule. However, it's important to note that while Heuer did indeed manufacture those other variants of the Monaco, it's becoming increasingly common to redial them as Heuers in order to cash in on the ~75-100% value increase. With a spare case and just a Heuer-signed bridge, your $3-4000 Edox/Diarex etc Monaco becomes a $10,000 Heuer Monaco - anecdotally I would say that the number 1 vintage Heuer model that we see fakes and redials of is the Monaco, no?

Which is why I prefer to stick to a "ho-hum" regular 1133B with the exact hallmarks of a regular production piece, rather than a "special" one that may not be real - until I myself become independently wealthy enough to have the time to go down the rabbit hole and research them to the ends of the earth!

In the specific case of the Edge, though, it's pretty well documented by this point to be a model considered for production, but not released in high numbers. Also very importantly, values for the Edge are low enough (<$1k I think?) that many collectors could purchase one and shake it off if it turned out to be fake. Not quite the same case with a possibly fake Monaco, being sold for around USD$ 10,000.... (which seems to be the average asking price these days for any and all Monacos, after the run-up a few years ago on 1133Bs)

 

Strange thing, i'm a very nostalgic person, but i don't see myself clamoring for old watches. I don't like smart watches either. Seeing as TAG HEUER is an entry luxury brand, shady things like this are bound to happen a lot. Either way, i wouldn't buy something expensive from someone i don't trust.

 

I have given it a good thought now for some days, let's say if for instance a Monaco collector finds it a.: nice and b.: a true addition to his private collection I wouldn't have any problems with it, I myself even did it with a TAG Heuer 1000 economy version (which I am never going to sell anyways) but if a fully blue dialed Monaco does show up in the economy/ trade, like for sale on eBay, Chrono24, or at auction houses or even in court I truly hope the experts will chime in and give it the correct description and treatment, or the floodgates are wide open so to say.
Just my opinion of course