Its kind of hard to tell from here. Looks like theres been some brazing soldering done to hold the nut in place. If so that will have to be removed. The initial damage to the stay nut was done more than likely by someone previously using a hammer and chisel to remove the nut. Thats easy to tell from the gouge marks on the nut. Its not a good place to have to do this; but Your option may wind up being to file or grind off the rough areas along the side of the nut; getting the high points off in order to use the right socket to remove it. The nut needs to be brought back close to original before trying to remove as this could well be one of the exceedingly hard to find parts.