This is a Parker Sac Duofold button filler in Blue, the same filling system as the original Duofold. Sac Duofold models were also available in striped Maroon, striped Green, and Black and featured a Visometer visible ink section. This 5 1/8 inch long pen is date coded 1941, as you can see from the highlighted imprint, and has three narrow cap bands. 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.