> aren't Schmidt nibs size 6 like Jowo and Bock?
Schmidt nibs are apparently made by Jowo and Bock - as so many other proprietary nibs too. I have no idea if the geometry is different or not.
Now that you mention it I also seem to recall that Schmidt nibs are made by Jowo (by Bock I'm not sure) but at any rate, Hari's picture clearly shows that, whoever is the manufacturer, the Schmidt #6 nib is different from either a Jowo or Bock #6.
It's shorter certainly.
Size 6 just means that the circumference in which the nib curve can be inscribed, has a diameter of 6mm.
The rest of the geometry of the nib can differ and that may make swapping nibs unfeasible.