This is a Parker Sac Duofold button filler in Black, the same filling system as the original Duofold. Sac Duofold models were also available in striped Blue, Maroon and Green. This 5 1/8 inch long pen is date coded 1941. Parker did not mark button fill pens with the Blue Diamond Guarantee and also gave them plainer unhallmarked gold nibs. The catalog says the Duofold nibs are 14 karat gold. They sold for $2.95 and the matching pencil was $2.00. Blue Diamond Vacumatic models were not cataloged in black. Sacless Duofold Vacumatics, Sacless Duofold Sub Deb Vacumatics and button fill pens were available in black.