Just spoken to a mate ,who has just been working on a c47 in France.He reckons there should be a small hole that has a small rubber `bung`,to prevent water ingress,but occasionally forgotten,so it corrodes,Best advice,use lots of dispersant/penetrant overnight,then try again ; if it wont budge,then a small disc cutter,down to the threads,rinse ,repeat...if you cut another one ,you can possibly spot weld the nut back `together`,if you need it again..(with a bit of filler)..