There was no brown Brooks B17 speical with copper plated rails in the old Brooks catalogue, so the brown Brooks B17 Speical saddles with copper plated rails were exclusively made for Brompton in the past.

However, according to the new catalogue, there are two more colours of Brooks B17 special saddle to choose from including the brown one.
http://www.brooksengland.com/catalog...g/B17+Special/

Therefore, no Brooks B17 Special saddle made exclusively for Brompton anymore.