The leather is supposed to be 4-5mm thick, but I've seen newer B17s with thickness ranging from 3.6-4.4mm (same piece of leather). Others have stated that their Pro or Imperial saddle is 4.0-4.5mm thick. I had a Flyer (B17 with spring) that's 4.3-4.8mm. My current Imperial narrow bought slightly used is 4.8-5.0mm (OM9 stamp on the back). This one appears to hold up better than the "thinner" B17 after 500 miles.
The Pro leather probably comes from the top of the cow where the leather is tougher than the sides.