This is an Empex Aqua Pen blue fountain pen c. 1952-1963. The Aqua Pen was the invention of Alex Goldman and Max Goldman, patented on March 31, 1959. Initially sold as the Universal Aqua Pen, by 1962 it was a Rohill Company product under the Empex brand name. It uses a replaceable ink cartridge that contains a sponge of dry ink in the base that is activated by adding water. The cartridge is inserted onto the back of the feed, as shown, the nib is dipped in clear water, and the cartridge is filled by squeezing it, mixing the water with the dry ink. The cartridge has SQUEEZE ONCE HERE TO FILL and patent number, 2,879,744 silk screened on the clear section. The patent contemplated three versions of this pen. The first would be a lever fill fountain pen with a dry ink sponge cartridge inside of the ink sac. The second version would be a leverless pen that would operate by squeezing a pliable end cap to compress the ink sac. The third version, which became this pen, has a replaceable dry ink cartridge unit. It's not known if the lever-fill or squeeze cap versions were made and existing examples are all like this one. Advertising claimed, "The Amazing New Aqua Pen . . . Just Fill It with Water and it Instantly Writes wIth Ink!" Known examples of the 5 1/8 inch long pens came with barrel colors in green, black, red, maroon, blue, and grey. The nib is gold plated stainless steel and the trim is gold plated. The pen sold for $2.98 in 1962 and $3.50 in April, 1963. Replacement ink activator cartridges were 35 cents, or later 3 for $1.00. The matching pencil was $1.00.