Much has been written about the World War I Trench Pen and the ink pellets that supplied it. While it seems an eminently practical idea, the present-day rarity of such pens suggests that it never really caught on. Several US manufacturers made them. Some, like the Parker and Moore versions held pellets in an extended blind cap, others like the Diamond Point held them in a chamber at the top of the cap. The Swan Military, holding the pellets in a compartment at the base of the barrel, was, so far as I can establish, the only British manufacturer of Trench Pens though pellets were made by several companies including Onoto.
Perhaps one reason why Trench Pens were not made in great numbers is because the provision of a pellet container in the pen is not that great an advantage when one sees the tiny boxes that pellets were sold in. It didn’t exactly take up a great amount of room in a soldier’s pack and it is likely that he already had an eyedropper filler which would work perfectly well with the pellet.
Powdered inks were already available and continued to be used in businesses and classrooms at least up until the 1950s. The pellet is just a step further. Some pellets dissolved to make an ounce of ink, too much just to fill one pen. It seems likely that they were just another ink delivery system already in use before World War I and adapted to suit military needs. The British Postmaster General prohibited the shipping of bottles of liquid ink to the front and it may well be that US authorities took a similar line. Tiny tins of World War I ink pellets can still be found today. Some manufacturers, such as Visconti, have made ink pellets in modern times.