In Need of Help About a Vintage De Ville 511.0404

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Hello to all,

I'm new to the forum. If I can manage it, I'd like to buy my first Omega watch these days. I've seen square case series 511.0404 with the Caliber 625, loved it and have been looking for a clean, genuine one for a while. I finally found this one, and based on information I've gathered from forums and other sites, I believe the parts are genuine in a way. However, something is bothering me. Although the case looks very clean and appears to be steel (no visible imperfections or scratches on corners or back), there's an inscription "plaque or g 20 microns" on the inside of the case back. Does this definitely indicate a yellow gold plating, or is it an alternative plating (like white gold or rhodium)? In the worst scenerio could this case belong to another watch so have I encountered a Frankenwatch? I would be very grateful for your help, thank you in advance.