I don’t have any information about Duofolds, but I can shed some light on your question regarding ink choice. I have seen some people report issues with Diamine Registrar’s Blue-Black. You can read about one example of that problem here, though I can’t tell you if it’s representative of people’s experiences with the ink, or if it was a freak accident. For an ink to do that to a modern gold nib scared me off it, though other modern iron-gall inks such as [[Rohrer and Klingner Salix]] and [[Scabiosa]] are much gentler on pens. I use both Salix and Scabiosa in modern gold-nibbed pens that I otherwise baby, with no problems.

Whether you want to put iron-gall inks in a vintage pen is up to you, obviously, but I personally wouldn’t go with the Diamine Registrar’s ink for a valuable pen, barring news of reformulation of some kind.

Some people only use brands that they consider “vintage-safe” in older or more valuable pens, but the question of “vintage-safe” inks does seem to vary from person to person. [[Waterman Serenity Blue]] is an option that Richard Binder swears by. If I remember correctly, that’s the ink he uses to test each of the pens he restores. If you want an ink that you’re pretty much guaranteed not to have to worry about, Waterman inks (barring the inks with red components—red, purple, and brown) are widely considered the safest ones around, but they have no water resistance to speak of, so there is a trade-off there.