Dating early-1920s - but I know nothing about Dennison cases. Definitely 9 carat.
David Boettcher's wonderful website
http://www.vintagewatchstraps.com/casemarks.php gives more info.
A high quality Omega movement, probably quality 'D' and known as a 'Leader'.
Difficult to tell whether it's a 19''' (diameter 42.3 mm) or 20''' (diameter 44.5 mm).
Recommend that you get it serviced!
You have PM. Tom
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