I have apparently sold the mirrors I had, but I checked the parts manual and thestuds aren't listed there. The only thing I could find was the mirrors and related connections but the nut size is listed as
Hex Nut Toothed M6 x 1 x 5 05 If that would help find a local replacement stud.
As I recall, they had an unthreaded portion about midway, probably to prevent bottoming out in the mirror.