Vintage Omega cases

Hi guys,
Nice to meet you everyone, I'm just starting to look into watches and very new to it so I'm hoping maybe you guys are able to teach me 😀 ... I came across a lot of omega watches which does not have neither signed crown nor back case, especially gold filled. I have read that there is a customised case for the US market? e.g. 'ROSS' that made cases for omega. If that's the case, I'm just wondering how can you tell whether it's original or a franken watch. Does the stamped inside the back case also indicate the movement it belongs to? (I'm just not quite sure how the Ref. number works for these customised cases) Does the watch become less dersirable for collectors due to this? Thank you so much in advance for any information 😁

That caseback looks good to me, I have one for a US market that is similar. I do not believe that they have reference numbers.

6288 is the reference number, not all of them are in the database though.

Use this site to find out more about the US collection.
https://www.omegawatches.com/vintage-watches?product_collection=3811&p=2

Or, use google to search for Omega L6288 & find at least 5 other watches with a cal 570 in the same case reference.

https://www.google.com.sg/search?q=...69i57j69i60.4846j0j8&sourceid=chrome&ie=UTF-8

Also, at least 1 of your pics is showing a different watch with a cal 354.

Thank you very much guys ... yh, I put pictures of two different watches just as examples to my questions, they both look original without any signed crown or backcase :s