The hardware is made of well-plated polished stainless steel, available in gold and silver. 

Bir-kin 20 and Bir-kin 25 are the same size, both for the Sellier Bir-kin Bag and the Retourne Bir-kin Bag;

Bir-kin 30, Bir-kin 35, Bir-kin 40 are the same size, both for the Sellier Bir-kin Bag and the Retourne Bir-kin Bag;