I'm pretty sure the thread size is M5.5 or M5. Can't recall which though. The metric screw that has an 8mm hex head I think is the right size. Actually, that would make it a 5mm Allen key wouldn't it? So maybe it is M5 after all. Try one and see.
Try one off a Japanese Strat like a Squier or similar.
95% certain it won't be an imperial size. That would be taking vintage accuracy a bit too far!
If you have absolutely no joy getting one then just get an imperial one, cut the threaded bit off and re-thread it with the correct die. As far as I remember the actual rod diameter is pretty similar. It'll be about half an inch shorter than usual but it'll work.