QUOTE
non-Lamy cartridges in a non Lamy pen
I'm assuming that's a typo and you mean, "international cartridges in a Lamy pen."
I, for one, have never been able to fit int'l cartridges to any Lamy. Strangely, I've been able to fit Lamy cartridges in Pelikanos, but not the other way around. I think the neck (where the cartridge meets the feed) of the Lamy cartridge is slightly smaller than Pelikan/Int'l. sizes.
I imagine there might be some tricky way to reverse-engineer the int'l cartridge so it
will fit a Lamy, but I've never been that ambitious.