Back in the 90's, one of the bike shops here was clearing out the basement. This pulled up a few B17N's with a "pebbled" surface. As a Brooks fan, I bought one to try.
It was indeed narrower. I don't recall the specifics, but it was probably narrower than a Pro. I didn't really like it and gave it to a buddy who was searching for a saddle that would work for him.

My impression is that the B17 is for somewhat upright riding positions, such as what you'd want on a touring bike. My Pro and Swift are on bikes with a lower riding position. I'm not sure if the narrow is for even lower positions, or just intended for people with narrower pelvic structures. Back in the 90's, I don't think there was a recognition that people had different pelvic widths.

How long has it been since Brooks made a B17N? ... or any models with the "N" suffix?

Steve in Peoria