Vintage Seamaster De Ville- Gold

This appears to be an early one and would tie in with my wife’s late Grandfather’s age. It’s running but fast, won’t wind easily so I haven’t tried. Simply a gentle movement gets it going. Date appears to change with second pull of the crown. Looking at hallmarks it appears to be gold, possibly 18k. Caseback is unusual in having Seamaster & Waterproof all in the central part. If someone can advise me how to remove caseback easily, I’ll have a look inside.
Here’s some images.

I’m a bit confused, there is a caseback surely?

Thank you so much for your help. As you said, off to a competent watchmaker.