Getting the outside screws out isn't too terrible. But there are two sets of two screws inside of the frame which are a real PITA.
The biggest problem is that I can't even get straight access with a screwdriver. The screwdriver has to go in at an angle and not have a good fit on the screw.
Im not even sure how the portlight was installed. Maybe they just lightly tightened these screws from an awkward angle, then over 4 decades they've frozen into place.
Note - I'm going to reinstall these portlights. In additional to traditionally sealing them, I also want to seal the edge between the deck and the liner with epoxy so if they ever do ever leak again, they'll leak directly into the cabin rather than disappearing and water showing up at random places in the boat.