Nib: Fine 14k Gold
1948 Parker 51 Cocoa Fountain Pen and Pencil set 1/10 12k Gold Filled, with crown engraved Cap, G/P Plain Arrow’ Clip with Pearl-Coloured Jewel. Filler Sleeve – Metal Aerometric Filler, One-Piece with Black Plastic End Cap / With matching cap-activated or "clicker" pencil 0.9 mm lead
The Parker "51" aerometrics
The MKII pens of course had a brand-new filling system that was to be adopted by virtually all Parkers in the future, the Aerometric.
Parker developed a sac in a tough plastic material, called Pli-Glass, that had a life expectancy of 30 years, but are now going on 60+. These sacs were also in a see through material which allowed one to see how much ink that was left in the pen. The sac was encapsuled inside a metal tube on which a metal pressure bar was mounted. To reveal this filling mechanism one had to unscrew the entire body cover from the section ring and back.