Hello and welcome.
The answer is "made for the American market". The cases were made in the US, by agents for Omega. They had a lot of latitude on how to design cases.
Secondly, I'm not sure if solid gold seamasters had the hippocampus (seahorse), and even if they did, this one could have been a 'presentation'watch - ie designed to have an engraving on the back for a retirement, etc.
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