I was a long time looking for suitable cartridges for my old cartridge fountain branded "Geha". Unfortunately, this required, familiar "P 6 - cartridge" is no longer to get for some time and the commercially commonly erhältllichen cartridge of one size fit here only conditionally.
So I had to search for quite some time and try out different cartridges. Most have convinced me it here. If I insets with the with "Lamy" label side for writing tip, everything works perfectly and my fountain pen writes the way I'm used since time immemorial.
