[tdk]

Most fountain pens have the nib and feed as one glued unit (confusingly, informally called the 'nib'), so you can't adjust the flow. Compare the 'nibs' at the top and bottom of this page. https://www.gouletpens.com/replacement-nibs/c/294 The ones at the bottom can only be used in Noodlers pens.

The Noodler's also have the feed made of cutable ebonite instead of cheaper plastic, so you can carve out a deeper ink channel if you want.

[pkd]

I have never used a fountain pen with a glued together nib and feed. The ones on the top of the page aren't glued together, they are the screw on type. These are generally found in hand-tuned pens (like the Edison Noveau) or premium pens (like Pelikan MX00 series).

You can still adjust the flow of the nib without tampering with the feed by just using a brass strip and running it through the slit in the nib (to increase the flow). I have never wanted to decrease the flow of a nib unit.

What you are saying is true though. Ebonite feeds have their advantages, like you can heat tune them as well to fit to a certain nib shape.