The first patent for a button-filling fountain pen was by John T. Davison in 1905 (US Patent 787,152), though it wasn’t mass-produced until later; Parker popularized the design in 1913, featuring a barrel end external button that pressed an internal pressure bar to flatten a rubber ink sac, creating a vacuum for filling. Moore received patent 1,346,045 in 1920 for a button filling fountain pen, and the patent was assigned to George Parker.
A standard pressure bar sits alongside the sac. Pushing on a button that protrudes from the barrel end results in the bar moving toward the center of the barrel, compressing the sac.