A good question and one that is not easy to answer. Firstly with mainstream lines, Omega introduced the applied logo in about 1953.
With De Villes, some Geneves and generics (models carrying only the brand name) Omega had several grades of dial including what they used to call "Luxury". As has already been hinted, the cheaper lines had cheaper to manufacture dials, but remember that's comparative to the particular line as opposed to the whole collection. Hence, a luxury dial with onyx inserts may not have the applied Omega logo in any of the three lines mentioned above, but on other occasions it will. It depends on grading. For example, a luxury dial may often have a coated base metal blank whereas the lesser grade would have paint on base metal.
This was different to other higher profile lines like the mainstream Seamaster and Constellation etc. which had symbol and logo and overall better quality dial. So the only logic can be applied to the De Ville (A product designed for the blue collar sector by Norman Morris) and lesser ranges is one of grading, and some of that grading is not visible to the eye.
Cheers
Desmond
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