as far a I search out
M9 size metric screw is not that common, or available at all
check with the workplace maintenance area with the shelf of metric and imperial size screws using a caliper
the metrics
m8 = diameter at 7.89mm at the teeth
m10 = diameter at 9.9mm
for the imperial
1/4" = obvious too small
5/16" = diameter @ 7.86mm
3/8" = diameter @ 9.33mm
my best bet is that the thread is an imperial inch type
and the size should be 3/8"
this based on two reason
1. M9 size threads is not not common or even a a standard size
2. seeing the actual M8 and M10 size, it is quite a big difference, as using a M8 is too loose while the M10 with the teeth > 9.5mm are not able to fit inside the 3/8 inch type thread