The TWSBI Eco uses the same nib and feed as the Diamond mini, Classic and Vac Mini models. However, there is no removable assembly like the Diamond mini so the nib and feed have to be pulled out of the body of the pen instead of unscrewing and swapping over a complete nib/feed/collar assembly. Therefore, we cannot describe these nibs as fully interchangeable. If you want to fit a Diamond mini nib to a Eco or Eco T, the nib has to be removed from the collar together with the feed. This task is quite difficult and can often result in damage to the feed. Therefore, we do not recommend changing nibs on the Eco model unless you are experienced with minor pen repairs.