Fountain pens are no more difficult to make than a rolleball or ballpoint. Two blanks on a mandrel between bushings that are turned and finished the same. Then the parts are pressed into the ends of the barrels. Most all kits are basically the same. If someone can make a ballpoint they can make any other kit. There is nothing mysterous about making fountain pens.
There were a couple of kits that used a step drill or drilling from both ends of the blank using different size bits instead of a step drill. The only one of those that are still around is the streamline el grande if I recall correctly. The streamline el grande is being discontinued, I think.
So, go get a fountain pen kit and get after it. All of the suggestions on which kit to use have been good ones. Remember: no difference!
Do a good turn daily!
Don