I used Seitz-Kreuznach Panthers Black in a Kaweco Sport with broad and extra-fine nibs.
When I put the broad nib to paper, and saw the ink’s shading, Panthers Black looked very familiar. That’s because, as detailed here, Panthers Black is extremely similar to Stipula Calamo Ebony Black as well as to Montlanc Mystery Black.
It’s a lighter black ink distinguished by its shading. It flows well in either the broad or the extra-fine nib. I would classify it as fairly wet.
This ink can feather a little on the worst quality paper, but not so much that I don’t use it on any paper. It doesn’t show or bleed through. It’s very easy to clean, and I use it in my delicate or higher maintenance pens without worry.
It is not waterproof, but close to washable. A remnant does remain on very absorbent paper after soaking, but not enough to be legible.
Here is a closeup of the writing sample next to Pelikan Brilliant Black, another black ink that’s not super dark but that has a different hue. If you click the photo to enlarge it you can see the distinct character of Panthers Black.
Here is paper towel chromatography of Seitz-Kreuznach Panthers Black, showing the dyes in the mixture.
Living outside the EU, I order Seitz-Kreuznach products from Seitz Global, which sells Seitz-Kreuznach ink in 100 ml glass bottles for $9.90 and in 20 ml glass bottles for $3.25. Short international cartridges are $2.19 for eight or $3.62 for 14. George Seitz kindly added this cartridge to an order I placed, so I could sample the ink, before they started selling the smaller bottles.