Not sure if they went on sale or not. As you say Arno, they're not in brochures - I had a look through the pricelists I have too, as they are often a good place to find more obscure watches that don't make it to the catalogues, but nothing in there either.

The reference on the sticker in this watch is quite interesting - unless I'm mistaken, it reads 74031 rather than the 74033 of the steel watches? So it might come before their release, which the Monaco table has as 1974.

Now, another introduction for 1974 was the PVD barrel Carrera 110.571 NC, so it looks as if the PVD Monaco predates the PVD Carrera (the serials are lower than I expect for 110.571s too).

So we have PVD Carreras going on general sale shortly after these Monacos, and evidenced in brochures and price lists, whereas the Monaco isn't. I begin to wonder whether the Monaco was a proof of concept of PVD coating for Heuer, and the lessons they learned went forward into the Carrera. Given the coating technology of the time, it was almost certainly easier to coat an oval watch with round edges than a square one with sharp edges.

There are a lot of detail similarities between the two watches:

Note the hands and the lume "stick" markers in particular.

So, all speculation of course, but I think it entirely possible that the Monaco was a true prototype, evaluated by Heuer staff, friends, family etc (the reference line on the sticker appears to say "1 pce", i.e. "1 piece" rather than the serial number we expect) with maybe a few sold to the general public and the end result for public consumption was the Carrera rather than a Monaco. Having said that, it's not at all easy to find 110.571s either...