Unusual case

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Hi all, came across this watch with a case back markings that I have never seen before on an omega watch. How common is this?

 
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Hi all, came across this watch with a case back markings that I have never seen before on an omega watch. How common is this?
Very common for Ref. 2421 only. Hardly seen on other Omega watches.

However, dial looks to have been recently refinished.
gatorcpa