I am just a bit higher than just a beginner, but my guess would be a mid-60's Seamaster.
What are the markings on the back? You can look it up in the Omega DB.
You will probably want to get it serviced, but be careful that no original parts are replaced. You'll want some advice from more experienced members. Do what you want obviously, but I'd keep it and remember your grandfather.
Good find.
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