Saw your post and decided to take a look at my Seamaster 200. I purchased it new in 1989 or 1990. I wore it for at least 15 years solidly until one day the bezel insert fell out and was lost. I tried to get it replaced but Omega said it was obsolete and offered to replace the complete case with the slightly newer version - which they did. It has the newer one piece case back compared to the two piece original like yours. Anyway just looked inside mine and found that it has no serial numbers at all.
I guess not surprising that the case back is not marked as it has been replaced but surprised that there is nothing on the movement. Furthermore also noticed that the movement does not even have an Omega logo on it ?!. Maybe they replaced that as well when they rebuilt it. It is the same ETA reference number as yours though.
Your hour and minute hands definitely look different to mine - which I believe to be original - though who knows perhaps they replaced those as well without telling me.
Chris
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