I watched a video on YouTube that explained that although the Centennial and International sections are nearly the same length (21 mm vs. 20.6 mm, respectively), the Centennial's nib is much longer (22.7 mm vs the International's 19.1mm).
The video is called "Pen tip #14 - Tell apart Parker International vs Centennial Duofold" (you can skip to the 5min 30sec part for the relevant content ).