I ended up getting the "Recessed Flat Panel 3-1/2" x 58" Plain Box Newel". I was retrofitting two existing newel posts, so I needed something a bit smaller than what you usually find with box newels. Stair Parts USA was the *only* place I could find a box newel in the size I wanted with a recessed flat panel. My shipment came before I expected it to and arrived in great condition.
Additionally, I got some incredible service over the phone on two separate occasions. Once was to notify me about expected shipping times during which I asked a few installation questions. It was so helpful and such a delight that I called again after the post arrived for another piece of advice. For an amateur like me, it was really comforting to know that I had some experts I could call for quick advice.