Pen, Pens, Pencils, Quills, Writing Implements

Is the spring loose/removable? Or is it fixed in the knurled section?

On mine the spring is fixed. You should be able to put the cartridge into the front section and feel the tension of the spring as you push onto it.

Although I can't see much inside, it doesn't look like it's a particularly special spring. If yours is missing, it might be worth trying to get a donor spring from an old click biro?