That is the Optima stub with Caran d’Ache Delicate Green ink.
Here’s a closeup of the line variation.
And just a little more.
The Optima stub has excellent line variation, which as a corollary means it feels a bit “crisper” to write with than some stubs. However, it still doesn’t require the care or precision of an italic or cursive italic nib. I’ve been writing with it very quickly and sloppily, both because that’s my normal style and because I wanted to figure out its limits. And it’s been absolutely flawless.
I have some other pens with stub nibs inked up, as well as a vintage Pelikan with OBB, so here’s a quick and dirty comparison. In order, we have the Optima stub (light green ink), a Lamy Safari with 1.1 mm stub (orange ink), a modern Pelikan with medium nib ground to a stub (blue ink), and a vintage Pelikan with OBB nib (dark green ink).
And a quick comparative writing sample with all four pens.
The Aurora stub appears wider on the downstroke than the Safari 1.1 mm, but it has thinner cross strokes. I’d guess that the Aurora stub probably writes about 1.2 to 1.3 mm wide.
I should also mention that mine is not super wet, so I’ve been able to use it with wetter inks. It’s shaping up to be a good everyday writer.
Fountain Pen Follies 
Laura, the pen looks outstanding. The stub nib with good line variation is most likely my favorite. I say most likely because I haven’t tried all kinds of nibs just yet. (I’ve almost tried all the variations) My nib of choice is a med. cursive stub. I am starting to think the Optima just may be a grail pen for me. I love that tiny little ink window too. I wish more pens had them!
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