Thank you all for the kind words.
Barry: The cap does not thread. It has an inner soft plastic liner that sort of "clips" onto the nib section.
As to how I did it, it's no secret at all. I bought some nibs from Elliot Landes. He has posted here with his nibs for sale. They come with a short brass section into which the nib section screws. This is how it comes apart to fill it. It also comes with the cap liner. The barrel is simply drilled to accept the short brass piece and the converter. I made up a pin vise to hold the drilled blank, and turned it down to match the OD of the nib section. The cap was a bit trickier. At the lower end, the ID needs to be very slightly bigger than the OD of the barrel. Then, you drill with decreasing bit sizes to accept the cap liner and a pin to hold it in place. I held the clip in place with a small set screw that I first glued into the end of the cap liner, then passed through a hole in the top of the cap. I glued it into place. It then did double duty as the pin securing the cap liner, and a stud to put the clip onto. Before putting on the clip, I drilled a small blank for the finial and threaded it onto the stud. I used a pin vise to hold it while I turned it down to the same size as the cap. Then, I simply unscrewed it, put on the clip and screwed it back on. I had done this with kit pens before, so it was not difficult.
I hope I've answered everyone's questions.