Someone ordered one of my brass fountain pens and wants to replace the nib with a Bock. The kit is the Commander from Woodturningz. They want to know what size the nib is - they referenced a No. 5 or No. 6 in their email. Does the size refer to tip size or the threads where the nib attaches to the pen? I only know that the nib is a medium point. I called Woodturningz, they don't know the size but say they use the same nib for all of their fountain pens. I'm hoping someone here knows.