Vintage Omega Seamaster, Genuine or Copy?

Hi my name is Harry,

I was recently handed down a Vintage Omega Sea master from Grandfather who passed away.
From what I know about the watch it was purchased when he retired around 30-35 years ago, however I don't know where it was bought nor do I have any documentation for the watch.

I was planning on taking it to omega to for a valuation but wanted to try and get some understanding of the model and authenticity before I did.

The strap isn't original as this was replaced by my grandfather.

On the back of the it says;
FAB.SUISSE
SWISSMADE
18K 0750
16222

Any information about the model or if the watch is/isn't authentic would be appreciated.

Thanks in advance.

Common fake.

Thank you for your responses, from information and images I'd found on the internet it was veering towards being a counterfeit. With it being my Grandfathers I never had any intention to sell but it was wether money was spent to get it to a better condition.
Now I know it's not worth the investment.