Where are you measuring to/from ? You say the 8mm thread one is the right length but I can't see the length mentioned ??
Best fittings 03C2-ML01 stainless HP fill valve 1/8bsp appears to be what you want & I'm tempted to say they will all be the same length because the valve & spring inside will be what determines the length.
One thread type might have more length inside whatever it fits to, compared to another.