The only difference between the P1 & P2 Aeroclam bike saddle bag is the contour on the top edge of the top section of the Aeroclam. The P1 is more concave whereas the P2 is flatter along the top edge.
While the P1 fits more saddle brands and models, the P2 fits 95% of the Selle Italia range but also some Specialized and Fabric saddles etc. So there is a bit of crossover between the P1 and P2 with both models fitting some saddles as nice as each other.
But, there are certain saddles that specifically fit only the P1 or P2 Aeroclam.
Capacity is exactly the same and all parts interchange.
Should you have any further questions, please don't hesitate to contact Pat via email firstname.lastname@example.org. Thank you!