These small motors (called pager motors) are used in micro airplanes, and usually the propellor is just a press to fit solution.
Make a 0.5 mm hole in the material you want to use, and press it on the shaft.
I guess it's the French system (mm)
A good one, I think the only country still using Inches is USA
Well the UK is mainly switched to metric in technical/engineering areas now except for legacy applications,
but things like beer units and road speed are still imperial (political conservatism). People
commonly use a mix of metric and imperial in everyday use now. SI will prevail everywhere in the long term I
think just because
1 - The convenience of almost no conversion factors needed (consider calculating horse-power from torque
in foot-pounds and RPM - in SI power = torque * frequency * 2pi
2 The fact electrical units are already SI and directly tied to the metre/newton/joule/watt. For instance farads
are a shorthand for columbs-squared-per-joule, volts are watts-per-amp, volts-per-metre = newtons-per-coulomb.