Thanks for your patience on a reply - we just got back from cycling Montanas Vacias in Spain!
I also was riding a B17 Narrow prior to either of these saddles and they both were too wide for me. Here is a website that describes the measurement specs for both Brooks and Selle Anatomicas:
https://cycletraveloverload.com/brooks-vs-selle-anatomica-which-is-the-best-saddle/Personal feeling riding all three of the saddles you are discussing:
- B17 Narrow fit me (skinny butt) well but has fallen into disrepair after years of wet muddy riding and is no longer supportive
- C17 felt wide and much firmer, but I don't weigh much (~155 lbs), the extra width was nice in a more upright position but still felt a bit too wide for me
- Selle Anatomica felt widest and most flexible, again the width was nice in more upright positions but generally just felt too wide for my sit bones
Looking at them side by side, the C17 and Selle Anatomica are very similar in width with the Selle being slightly wider, and both are ~1cm wider than the B17 Narrow.
Hope that helps and holler with more questions!