Prototype dials is a claim we see quite often when something is unusual or doesn't quite fit the norm. Unfortunately, it makes it much more difficult to prove anything.

But let's work from what we do have.

Prototype dials at the time, and often right through to the present day, came from the dial maker rather than the watch manufacturer. Sometimes at the maker's behest, sometimes more speculatively from the dial specialist. You might recall that, in similar fashion, the Monaco's case was offered to Heuer rather than being something they specified. The dial maker would put decent effort into it, after all they were trying to sell something to the watch brand, whether fulfilling an order or more speculatively.

Now, I believe all the Monaco's dials were from Singer. Who obviously had cliché elements at the time of this dial that were already in use for Heuer. Those had (admittedly slight) serif elements on the Monaco text, which we don't see in this dial. They also had a very good to perfect (quality would marginally degrade over the lifetime of the cliché) rendition of the Heuer shield, whereas the one in this watch is a bit off. If this dial was from Singer, they would use existing printing elements to show it off to best effect and it wouldn't have come out looking like that. They are trying to sell the dial to Heuer after all.

So, if not Singer, might it have come from another dial maker? Potentially, though we're in an area of greater uncertainty. I still feel their proposal would have shown greater accuracy in the areas mentioned above, a better rendition of the Heuer shield and serifs on the Monaco text. Those elements weren't copyrighted to Singer. Any dial maker puts out their best effort, else they don't make the sale. Do you feel the dial in question represents a best effort?

As to why it's on the OTD site? There was a time, 10+ years ago, when digital photos were significantly lower quality than they are now and the audience were arguably more accepting of anything new and exciting and less analytical and sometimes critical than they are now. There's potentially stuff that wouldn't make the cut now and everyone should do their due diligence before buying, rather than simply relying on the presence or absence of a photo here.