The 58C is a very popular model from the Airmail Pen Company. You can read about it here.
The 58C CARTRIDGE is a new stock model with a two in one filling. If takes short as well as long international cartridges. You can directly fill ink in the barrel in eye dropper mode as well. The K5 K7 converters with me would not enter the barrel. If you have narrower converters probably a converter will fit too.
the stock F nib is pretty smooth. There is no nib unit as such. The nib and feed appear friction fit in the section. The end of the section has insitu moulded cartridge nipple etc. This allows the pen to be very economically priced and eliminates burps etc. A long term use will reveal how effective the feed really is.
that’s a pelikan Gtp/5 cartridge. Fits perfectly. A short blue ink cartridge comes with the pen.
Compared to the ED only 58C. You can notice the cartridge version has a longer section.
Cheers
hari