Ah old fella I am guessing you must have seen this bike in the flesh. You even had me looking for a black one but I am fairly sure a brown one would be more in keeping with the colour scheme on the bike.
I go along with oldfella - a Brooks Team Pro, copper rivets and rails, would be the perfect saddle - I am undecided whether brown or black but I have two bikes with black Team Pros, so I'll plump for the black version too - at the end of the day, it's all about comfort and after a century or so, the colour is usually irrelevant