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Aerometric 51's - Parker introduced a new filler system to the 51 line in 1948. The ink sac is held in a metal tube with a pressure bar that is pressed to fill the pen. The body of the pen screws on to cover the filler. They also changed the ink sac from rubber to a transparent plastic. These fillers are excellent and rarely fail.

Aerometric 51 pens come in black, midnight blue, navy gray, cocoa, burgundy, and teal blue. Forest green and plum were added later as well as the stainless Flighter, gold filled Signet, and solid 14K gold Presidential pens. 51's were also made in England in black, teal blue, navy gray, burgundy, turquoise blue and maroon. Solid gold Presidential pens were made in solid 9K and 18K gold. There is also rolled silver Insignia and a rolled gold Insignia models.

The first Aerometric pens made in 1948 came with the older blue diamond pocket clip but rest come with the arrow pocket clip. The caps are in lusterloy, stainless steel, sterling silver, gold filled metal, and solid gold.