An Esterbrook nib. The Master Durachrome was the premier Esterbrook nib from the 1930s through to the 1950s. These nibs fit all the Esterbrook pens from the 1930s onward. They also fit some Osmiroid pens from the 1950s and 1960s, and also some Burnham and Venus pens. Completely undamaged, hardly used.
I have tested this nib and it works very well with good smooth line.