I would vote for M10x1.25 (10 mm diameter 1.25 mm pitch) this is the most common 10 mm thread used on Japanese bikes and cars
Regular coarse thread is M10x1.50, and on European stuff, like Brembo calipers/banjo bolts it is M10x1.0
Sort of like UNC, UNF and UNFF for you Imperial guys!
I actually haven't checked on my XS, but at least on my RD, I believe the LH mirror had LH thread, so the wind pressure will not be able to loosen the mirror. Best to check that before attempting to fit a mirror or chase the threads with a tap.