In fact, installing a Schmidt type International converter is a quite hectic business if it is not the 'Kaweco' converter! You should also update the product description as well.
If you just try to fit in any Schmidt converter, Chinese International ones doesn't even fit due to their nozzle diameter, the fitting is not tight at all. If you try to ink with this state, the converter will not be inked fully due to lack of good sealing at the section-converter area.
It turns out that the section's inner wall has metal thread. Thus, you cannot use 'any' international converters at all. The feed's converter/cartridge fit might be the international size, but the inner wall thread of the section actually prevents many international standard (Schmidt type) converters to fit.
Normally, Kaweco's International standard converters have those outer wall threads to fit into the section's inner wall thread.
I'm not sure this is the case on 250 model. But 060 model has those inner wall trouble. You can fit any converter if you can risk damaging converter's surface by forcefully screw into the section. The metal inner threads will scrape through the converter's surface. It kind of works but not a good solution.
My unit with EF works quite ok with the 'forced' fitting. But getting a 'Kaweco' International Converter is recommended.