That 6.3 mm dimension is within 0.05mm of 1/4", and the 1mm pitch is 25.4 tpi. So, it may be worth trying one of the following British threads:
- 1/4BSF is 26 tpi with 55 degree thread angle,
- 1/4CEI is 26 tpi with a 60 degree thread angle.
Both of the above threads have radiused crests...