First make sure you are using a good hollow ground gunsmithing screwdriver that completely fills the slot. This is no place for a cheap common ill fitting tool that will break or cam out of the slot and bugger the screw. Second, allow plenty of time for your penetrating oil to get into those 80 year old threads. I have not had good luck with Kroil though some swear by it. I have found that it often will not penetrate more than a couple threads deep, even with long soaks. Don't be afraid to try something else. You could rap on the end of your well fitting screwdriver to help break the bond of the threads, or apply the tip of a soldering iron or heated copper rod to get some differential expansion of the male and female threads. Patience is most important. When you get impatient to get this frozen screw out is when you will break it or damage the screw or gun. I have soaked stubborn screws for months until they finally came free.
