Wahlnut

Numbered Adjustable nibs were
3,5,6,7,9,10. No 0,1,2,4,or 8 made. There were physical construction issues that make snything smaller than a 3 problematic. Nib Shan size defines section bore or vice versa. Too many shank sizes cause expensive production issues. You will find that #6 and #9 adjustables have necked down shanks. All Adjustable nibs were designed to go from very fine or fine to broad. They were part and parcel to the idea that one nib could satisfy many writers. That was a developmental trend at WAHL and Ecersharp ever since the Personal Point. If you search here and on FON etc for discussions about these concepts you'll find a lot of information in the virtual pen library.