The Stern company made many of these. The artists would make sketches and the dial maker would shop them at the various brands. Some were true comission mission pieces, like the Omega observatory ref OT2699 and some Coat of Arm models, but many could be shared between various brands - that's why popular motifs like flowers, mythical creatures, animals, patterns, maps, sail boats etc were popular. The artists were paid 50 - 150 CHF for each dial they made for the company (obviously Stern would then sell the dials to the watch companies at a mark up). The back of these dials were counter enameled (to prevent warping) and the Stern logo, a star, would be stamped on the back, together with a customer code (number). The artist would sometimes also etch their name onto the back to of the dial.
Re second example specifically, I suspect the Omega text and minute track were cleaned off. These text were padded printed and prone to complete removal when cleaned. I'm not sure why "Seamaster" doesn't appear, as this would have been painted onto the scene and fired, thus becoming a part of the artwork (which is essentially glass, so can't be removed). In the case of Omega, both the nepture ref OT2520 and observatory were equipped with chronometer grade movements, so time keeping would have been important. It makes little sense not to have a minute track in that case.
Re third example specifically, it is unfortunately not the work of Nelly Richards. The use of colors are different, and the details of execution is sub-par (see details in the hand holding the trident, details of the shadow areas of the bicep and under the pectoral muscles, for example). It could be a later creation, as some of the enamel paints made in the 1950s can no longer be sold due to toxicity concerns, or it could be the stylistic choice of another arists - red being not red and black not being black. The dial still looks like cloisonne enamel.
Re fourth example specifically, it is a complete fake and a pretty crude one at that. If you look at that dial closely, the word CHRONOMETER isn't even spelt correctly, it should CHRONOMETRE with an accent over the first E. I don't believe the dial is even cloisonne eneml. I honestly don't know why Antiquorum would go through the trouble of selling this.
Also note both the third and forth example use incorrect applied markers for this dial, which could be UNIQUE to the ref OT2520.
I had a chance to see the Neptune in the Omega museum in Jan 2025. It is as amazing in person as shown in the picture above. I think everyone should go and see at least one of these 1950s cloisonne enamel dials at least once.
One final point, the last Rolex ref 4645 would be an amaing feat of dial construction if all the applied markers are set into the dial with dial feet. This could be easily done if the feet was set into the outer rim of the dial, which is made of 18K gold, but extraordinary difficult if they were set into the cloisonne enamel, which in essence requires drillng into glass in 12 positions without cracking the whole thing. I therefore suspect applied markers re glued on, rather than set into the dial. However, if anyone else knows otherwise, I'd be interested to learn.