A while back, a penmaker on the IAP and Fountain Pen Network organized a group-buy of taps for the Esterbrook renew-point fountain pen nib units, and I joined in.
It took me a couple of attempts to make this work, but I finally managed to make a nib section that holds the nib unit in one end and a cartridge-converter in the other, and hasn't let go of either in a couple of days testing.
I decided to try making the clip-ring hidden, but experimenting on some scrap acrylic pieces taught me that none of my existing tools were fine enough to cut the groove that the ring seats in. I have a couple of old files that I'd repurposed as scrapers, I ground down the tang of one of these into a very narrow square-end scraper -- it did the job nicely.
Cap-barrel threads are 12mm x 0.75, barrel-section threads 9mm x 0.75. The cap posts (but I forgot to take a photo of it posted). Thanks for looking.