Vintage Omega With Two Serial Numbers: ID Help

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I'm considering purchasing a vintage Omega Seamaster from 1968. It has two serial numbers listed with its certification. One is clearly the serial number that is engraved on the movement. Why would there be a second 8-digit serial number? And to be clear, this second serial number is not its reference number.
 
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Pictures. We want pictures
 
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I agree, pictures would be very helpful!

Though, I suspect it will very likely be that both a movement number and a case number have been given