That's a pretty big bolt, that would be like a 7/16 in SAE, are you sure it's not a 10mm?
I just checked with mcmaster.com and the size you requested is not available in 1.25
thread, 1.75 for 12mm and 1.5 for 10mm.
I would not think they put a special thread on the mount, a standard coarse metric
would be the most reasonable for that location, reason being is the coarse threads are
not as susceptible to corrosion and dirt damage on the underside of a vehicle
Wild