Barrel slides open to reveal squeeze filler

Laughlin fountain pens are perhaps the best known early fountian pen to use this style of squeeze filler or thumb filler system for fountain pens. It is seen on other vintage fountain pens as well, notably LeBoeuf. As opposed to a standard “sleeve filler” fountain pen in which an outer sleeve which covers a hole in the barrel that opens to access the squeeze bar for the sac, the barrel itself extends and slides open to reveal the squeeze filler. It is functionally a sleeve filler, but more elegant in that it maintains a uniform barrel width.