That means you can use a Dupont cartridge in a Mont Blanc pen pretty easily. The full size cartridges take up the full length of the pen barrel. The half size cartridges can often be sorted back-to-back, meaning your pen has a onboard refill if your working cartridge runs out of ink.
I was under the impression that Mont Blanc used a proprietary cartridge system that doesn't play well with other kinds.
Personally, I like the half cartridges as you always have a backup in the pen and I always carry an extra pack in my folio to make sure that I'm never out of ink.
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I have a Lamy 2000 EF that I use occasionally, and I do like it. Wonderful pen. Lamy seems to me to be very precisely engineered in ways that I think of other pens as well crafted, if that makes sense. There's something terrifically utilitarian about it in the way that form follows function for Lamy. Not that they're not well designed or handsome pens (they certainly are).
Lamy are a bit like German cars in their clinicality, I've found. They're excellent at what they do, but sometimes they lack that connection either to the road, or in this case the paper and the overall 'feel' of the experience. But, that being said, Lamy make great cheap, daily use fountain pens that are a good gateway to the fountain pen rabbit hole.