It's threaded. Soak in warm water might break up whatever crud (or shellac) is gluing the nib unit into the section.
A few years ago there was a group buy on the tap to make a section that works with Esterbrook Renew-Point nib units. I've made a few sections that take them - it does get a bit tricky because there is no nipple on the rear end of the unit like a Bock or JoWo nib unit would have, so you have to create something yourself if you want to use a cartridge/converter rather than use an ink sac or eye-dropper the barrel.
[edit: search on eBay and you'll find pix of what the Renew-Point nib unit looks like when it isn't in the section.]