Be Aware that some of the KWZI Iron Gall Inks had something in them that did NOT react well with the TWSBI DEMONSTRATOR Pens. The interaction caused something like etching on the clear barrels. I believe KWZI subsequently modified something in the ink so this wouldn't happen. Which ink is this? Do a google search of the internet to learn more.
https://vanness1938.com/collections/kwz-iron-gall-inks