I read here that some new Omega watches from "certain illegitimate sources" have the serial number inside instead of on the back of the case. I purchased a new DeVille 4861.31.32 caliber 2500 from a reputable but "gray" source. It has a transparent back and hence the serial number is engraved inside in tiny gold numbers close to the "27 JEWELS" engraving. I saw a review of the caliber 2500 (link provided) and the serial number in the photograph is also inside in the same location as mine.
So I hope it is a fair assumption that the general rule for serial number placement does not apply to watches with transparent backs.
The watch came with the Omega credit-card sized Chronometer Certificate with model and same serial number, and the Pictograms card also with model number (and caliber number). All I forfeited by buying "gray" was Omega's warranty, but thankfully it is running perfectly.
