Re: Installing Box Newel Posts on pre-fab stairs
The way to do this "for the ages" is with threaded rod that is set into framing/blocking below, and bolted to a flat block inside the top of the newel that is well-fastened (best if dadoed in) so the tightening nut and washer sits on top just below the cap. The cap should be made so it's a press fit, or threads onto the top extension of the threaded stud, so it is removable for access to tighten the nut should the need arise. The compression member (rod) keeps it vertical and strong. Nails, glue, and lag bolts can become loose, the rod system is serviceable because the cap comes off for access.
Remove not the ancient landmark, which your fathers have set.