The Esterbrook #9128 nib is one of my favorite Esterbrook nibs. The box calls the nib a “Flexible Extra Fine” and that it’s for “Flexible Writing.” The Paul Hoban book has a pamphlet reproduction from 1955 that lists the nib as a “Extra Fine Flexible.”
It’s a steel nib and it’s not so flexible it will do gymnastics when it writes. But that’s OK since flexible nibs are wasted on my writing abilities. I love the nib because it’s nice and thin and also has just a bit of spring to it. Sometimes I like writing with a nail, sometimes I like a little spring. This nib is perfect when I want the latter.
Like other 9xxx series nibs the Esterbrook #9128 has iridium tipping and was called a “Master DuraCrome” nib by Esterbrook.
My particular nib has a nice smooth extra fine point that’s smooth even without allowing for its age. I was especially surprised such a thin nib, and one that’s probably 50 years old, would be so smooth. Smooth is relative because like most nibs this thin it does require a light touch since the thin nib can easily stab the paper. Course or textured paper, such a linen or cotton paper, also gets stabbed by the nib, even with a light touch.
Inkophile compares 9128 nibs. Mine has 9128 written across the nib, and while I can’t compare to the other style, I would say mine is one of the ones with less flex. It is also certainly the finest Esterbrook nib that I have so it seems consistent with Inkophile’s findings.