I have ran into the same problem with very stuck screws in century or more old guns. If made no progress after an extended penetrating oil soak I use this trick.
I grind the tip of a cheap soldering iron to fit the slot tightly. Every afternoon set up the iron in the slot and let it sit for 15 minutes or so to heat the entire screw all the way down the threads. Then put a few drops of oil around the head. As the screw cools it pulls the oil down in around the threads. After a week or so of this process every day they usually come out like they were installed yesterday.
