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Well for starters the case has nothing to do with a SM300, or with any other Omega. Just google the earlier ref SM300 (2913 or 14755) and have a close look at examples. The differences will be quite obvious.
It is a genuine omega movement. Case is not omega. Omega never put anything like that on their casebacks.
I messaged the seller warning him he had a fake or franken and his response was that it was all original and very very rare. Uh huh.
The seller sent m a long rant where he goes into how "most" of it is original so it's fine. Uh huh. Sure. Reported.