Cases were probably scrapped for the gold content.
Gold plated cases tend to wear out and look terrible once the base metal starts showing.
This has been going on for decades, so there are a lot more orphaned movements than dials. There is a lot of competion for the remaining cases.
https://www.omegawatches.com/vintage-watches is a good place to start learning the reference numbers.
There are a lot of 620 movements out in the wild. These 18mm movements were used in ladies watches and gents dress watches. I do not have as much experience with 510. That is a 25mm movement, also looks like used for dress watches.
Since these do not have sweep seconds (or any seconds hand) they are not too popular.
27.5 is a common dial size for smaller watches. There are so many dial variations, that finding a similar one is more luck than skill. There is not much reference at hand other than old adverts and catalogs, which could use retouched and even hand drawn images.
Curiously I have a 620 stainless steel case with no dial or bezel. Finding a decent dial for a decent price is difficult. There is a bit of competion for the good stuff, so the overpriced examples (often with visible damage.) sit unsold online.
