Difference is the frame shape, and the Leather dies to shape the leather is made to go with that .

B17 is flatter across the back, Road racers used the Pro.
The Traditional Campag Seat posts , are perfect with Brooks saddles.

That is why there was a combination wrench was made By Campag, open end for Brooks tension nut,
boxed end for the clamping/angle adjustment bolts on top of the <C> seat post ..