that would be m5 x 0.8 mm then .
unless rob's got some super exotic bolts.
here's a thought though .......
I bought a domi off a nice man, which had a piece of cotton wedged down between the trigger and the plastic button , it was acting as a thread locking device .
the buttons can get quite loose and swivel around.
so even if you do get the wrong pitch on the thread you might even benefit from the tight fit.
err... only mismatch the threads if you're using a plastic button though