I wonder if anyone in this forum can help me with any information about this watch I inherited from my grandfather. He was Swedish and died in 1983 and I'm pretty sure he would have bought it new. I don't really have any more information about it.
You should have it opened by a watchmaker, and take a picture of the movement and inside back. Movement should be one of the 26x series, the serial number will tell us the production date (late fifties I would guess)
Those lugs (twisted lugs, or bombé lugs) are pretty distinctive and uncommon for Omega dress watches from that era. A quick image search makes me guess it's a reference 2791, but I'm sure there are other possibilities. https://www.omegawatches.com/en-us/watch-omega-seamaster-omega-ko-2791
A little more research shows my guess is wrong since that reference has a press in case back rather than screw in (and it looks like a slightly different lug style). In any case, @Franco's advice is the way to go. Very nice heirloom, by the way.