Correct, registrars ink is needed to sign the register. It’s a particle ink rather than a solution, is mildly acidic ( sulphuric acid), will eventually rot nibs and will easily clog nib feeders. Iron gall ink as it’s otherwise known is believed not to fade for 400 years, hence the requirement to use the wretched stuff. Clerical Stationery Supplies, based in Haverhill used to be the only place that the home office got the stuff from, several registrars got a bit upset when I told them you could get it from any distributor of Diamine ink, as they are the supplier to the CSS.