Fretboard is glued on to the neck blank.
The other neck is for a different bass. Same scale length, but a different body style and longer fingerboard.
Unfortunately I only have one length of truss rod and it looks like a headless (because I have to cut away part of the neck past the nut to recess the Hipshot head piece), 33" scale neck with only 21 frets is just a little too short for these rods. It doesn't cause a functional issue, but I would have preferred that the little flat bar on the top of the rod was totally hidden by the fretboard.