Double Spares
From Penpedia
Many Pilot Capless pen models made before the 1970s required special ink cartridges called Double Spares. Double Spares were shorter and narrower than standard cartridges, and had smaller openings. They were designed so that you could keep two cartridges in a metal sleeve in the barrel, which is where it derives its name. Double Spares were discontinued in the 1970s.
To the right is a photo of a current Pilot/Namiki ink cartridge (a.k.a. Single Spare). In front of it is the shorter Double Spare cartridge.
Because the mouth diameter of the old and current cartridges is different, old model Capless pens cannot use the new cartridges. However, Pilot has produced a special double spare ink converter which fits these old pens, eliminating the need for cartridges.
