Waboo,
I don't have that answer for you. I know that most manufacturers back in the day did not use standard thread paterns or pitch, and I don't have a gauge to measure them.
Are the threads buggered up?
Is the screw still in the stock? If so, I've backed the screw all the way out of the metal ferrule in the stock on several of mine to clean up the threads, doesn't work too bad. Can be kinda snug though, had to use a fat old screwdriver on the last one.
If the screw is aleady out of the stock, you could always take it to Homer D. They have a gauge built into their screw isle. Just try it gently, if it threads in without binding, go to the tool 'corral' and find the appropriate die to chase the threads with.
If that doesn't work, I'll look to see if numerich has them. Wouldn't be original, just a newly made to original specs.