I have a brand new b17 brooks that I was going to put onto a 72 px-10 but I just realized that the saddle rails are not the same distance apart.

I'm planning on keeping the original simplex seatpost that requires a larger distance between the rails.

Is this another case of weird french sizing? I guess so, but I wasn't able to find anything in the forums about it after a quick search.

I guess I figured that saddle rails were a pretty universal standard but the more I dig into bike parts the more I'm not surprised by this kind of thing.

Overall question, Is there a lot of variables in saddle rails?