What if I can't buy a matching ink bag for my pen?

If you must have a German pen, the first consideration is that there is a treasure on the European standard, which is usually not very expensive.

In principle, as long as the interface caliber is the same, it is no problem to use other brands of ink cartridges (the pen is a special type, if the pen body is thinner/shorter than other brands, that is another matter), but please clean the pen before switching to prevent the ink of different brands from mixing and blocking the pen.

The safest way is to inject ink yourself, which can avoid considering whether it is applicable or not. Theoretically, an ink bag can be reused many times by injecting ink, which is more environmentally friendly.