I have wanted a Duofold style every day carry pen because I am afraid to carry my vintage one with me, so this is what I came up with.
I used a vintage Parker clip and installed a modern Schmidt nib in it. 13mm triple start cap threads. It is 15mm in the middle with a taper to the 13mm at the blind cap and the threads. Cap is just shy of 15mm.
So I was very pleased at how well it all turned out. I just wish the original Duofold had a screw in section so I could have just used and swapped it over it but it is what it is.