Very rare indeed and with an interesting story.
Actually, this one watch from a friend you have pictured here is probably the only one to have surfaced so far. In 15 years of closely monitoring all well known markets, it is the first and only time I have ever seen it.
So absolutely rare 'in the metal'. So much more than its European Chronometer cousins, even cross-hair / pink gold ones.
However, it is well documented in advertising, published in many newspapers in 1958 but also as early as 1957:
2 simple ads from 1957
and 2 from 1958 :
1958: Shown with Commodore Bouman, the Dutch Master of the "Nieuw Amsterdam", flagship of Holland-America Line
And finally, another add from the same series of the 'men who command'. It takes the same storyline and also highlights the 125th anniversary.
However, the pictured watch is a non-chronometer version, luxury Master mariner, so is of a different model than the elusive 18k LeCoultre Chronometer (it also has a different case and is 14k) .
I believe this is what Nicolas you refer to as '2 dial variants are known to exist'. But I tend to think that only the dial of the actual watch you photographed was produced for the 18k chronometer.
Nonetheless, here is this other advert (this watch, as opposed to the Chronometer, I have seen a few example of):