Seamaster 166.0020 clarification helps

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Hello gentlemen, I need some advices on a vintage omega seamaster 166.0020. The case back of this one looks really different from what I saw from others so far, usually there is omega logo below the seahorse, sometimes with water proof written on the outer rim of the logo. Is this a sign of it being fake?

 
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Perfectly genuine, no issues with the case (or the rest of the watch).