Vacuum-Fil

What “Vacuum-Fil” means here

Sheaffer's plunger filler of the 1930s and 1940s (also Onoto's): one stroke of the plunger creates a vacuum that fills the whole barrel. The packing seal is the failure point. He no longer restores "Sheaffer piston fillers" (pen-repairs page), so these are sold restored but not taken in for repair.