This is an idea that I have been playing around with for a bit now.
I wanted a pen that the body was all one piece but was also a bulb filler.
I made up the section and the ink reservoir with the bulb screws into the window part and the section screws into the other end. The window part also has the cap as well as the body threads on it as well. This small piece has two external threads and two internal threads on it.
I also set up my taper attachment to cut the tapers on this one. It is a boring head with about 15mm adjustment on it. I mounted a live tail stock head on it so now all I have to do is mount the piece in my lathe and do straight cuts but I am now getting a nice taper on my part.