Hi "J".
I think POV has nailed most of the Info that we can give you at the moment.
I wouldn't class it as a Seamaster. Seamasters are clearly marked so, but in the 60's Omega made a number of very similar looking models.
As you've seen there are a number of "could be's" or "almost's" in the Catalogues of that period.
In order to give you any detailed Info we need to know the Case Ref.number. That is stamped on the inside of the rear case cover.
If the seller knows the movement number he may have had the movemnet out & jotted down the Case Ref number.
The Case Ref number should also be present on the Paperwork somewhere.
It will most likely be a number like 165.xxx, 365.xxx , 162.xxx or 362.xxx
Before you replace the crystal , you might want to try polishing it. Brasso, Displex or Polywatch all deliver good results & Brasso will also brighten up the case a bit.
I have no doubts that it is genuine. everything about the watch looks "right". Unfortunately, it is not a model that has a huge market, which , fortunatley, means it is of no interest to the fakers out there.
If you have time I'd be interested in more Info & pics of the box /paperwork.
If you're not sure about posting the pics, feel free to send them to me & I'll post/host them for you.
All the Best & Seasons greetings
Steve