The Phosbronze untipped flex nib is an option for fountain pens sold by Osprey Pens. Phosphor Bronze is a copper alloy which has corrosion resistance, strength of copper, while phosphorus increases its wear resistance and stiffness.

The nib can be purchased as a single nib with an ink reservoir attachment. Price is US $12.

This allows the nib to be used with a dip pen.

That little ink reservoir can hold enough ink to use the pen for a long period time without reload. Shown above is A4 paper. I had to stop because I'm out of space, not out of ink.

The nib can also be purchased as a nib unit which comes with the feed, section and screw-on piston ink convertor. Price is US $18.

The nib unit can be purchased separately or as an option with fountain pens from Osprey Pens.

If you buy the nib unit at a later date, make sure to get the nib unit designed to work with your fountain pen model. The nib unit is not interchangeable with different fountain pen models.

The nib unit can be disassembled completely easily. To push the feed & nib out, it's best to use a stick to push them out of the section.

The section is designed specifically so that the feed and nib can only fit in a certain way. This helps to align the nib and feed properly when assembling the nib unit.

That's how the nib looks when I first dismantled it. Not sure why it looked that way when it's unused.

The nib is untipped. There's no bulbous tip you commonly see with fountain pen nibs.

The stiffness of phosbronze is quite similar to common steel flex nibs. Main difference here is the phosbronze nib is untipped and hence slightly smoother.

The line variation possible is Fine to Broad.

Since the pen tip is not extra fine or sharp, the pen nib can be pushed upwards without digging into the paper.

This nib is good for quick sketching because the nib is smooth and doesn't cut into the paper surface.

When used as a dip pen, there could be ink blobs when drawing too fast. As long as your movements are more controlled, ink blobs can be avoided.

Ink flow is fantastic. To get more consistent ink flow and performance, it's better to use the nib together with a fountain pen.

Since the nib has flex, you can get some line variation depending on the pressure you apply.

Performance of the phosbronze and steel flex nib is quite similar, just that phosbronze is untipped and hence smoother on paper which makes it better for drawing purposes. Phosbronze is not more flexible than steel nibs though.

I'm not into calligraphy so I can't say how good the nib is for calligraphy work except to say the nib can flex.

The pricing of US $12 (nib) and $18 (nib unit) is very reasonable, and hence it's worth the money, and it's worth getting one to test.

You can find the Osprey Pens phosbronze nib from their online store (affiliate link):
https://www.ospreypens.com/discount/teohyc?aff=39