Help! Omega moon watch. Case back , real or fake?

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Hi guys! I have a case back of moon watch, do not know how to real or fake? Please look it for me, thanks
 
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The caseback is basically genuine, but has been "customized". The original writing on the outer side was ground away in order to be able to engrave an advertisement or dedication or whatever these "20 anos" stand for.
 
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I am probably wrong, but it looks like a 20 year service presentation.