I had this on the improved model B ("screwdriver sos thread")--I managed to get a screwdriver from Halfords but it was a bit thin in the blade and the shaft really so a proper fitting driver would be a real help. I removed the trigger guard and trigger and put some plus gas in the hole where the bolt goes and left it overnight. I inserted the screwdriver into the stock and put a small bart through the handle and kept a bit of pressure on. Once it moved, I tightened it again before trying to undo it some more. I did this until it came most of the way and then when the stock came off and only the fit of the stock on the bolt was holding it, tapped it out. There was a fair bit of crud on the bolt which was why it was tight. Someone suggested Beeswax to lubicate the bolt and I suppose you could try getting a bit of that down to the edges of the bolt head. Ive been thinking about this now and maybe if you can squirt some furniture polish down to the side of the bolt, this might help it "slide" across the wood. The bolt on mine is thick at each end for about an inch and tapers to the middle, so that is what is holding it in the stock once the thread is undone.