The Supra is very clever. It has a removable mid-section, giving you the option to extend the barrel into a full-size pen.
At full size, the standard Kaweco Cartridge Converter will fit comfortably inside. If you prefer a shorter, lighter feel, then you can remove the middle section for a neater, more portable pen.
At this size, the standard international Kaweco Cartridges will fit perfectly.
The cap of the pen can be screwed on securely at the end while in use, which you'll find much more comfortable when using the pen without the midsection.
Depending on how you configure the Supra, you can find the perfect combination for your hand. The difference in size, length, and weight will change the feel as you write, so it's worth trying the pen in different combinations.
The beautifully large 250 Bock nib is made from stainless steel and looks suitably impressive when compared to the Kaweco Sport or Perkeo. It's a smooth writing, fine nib with consistent ink flow. This pen is truly a pleasure to use.