Blue sweet iron mouth on stainless steel, with a copper/brass lozenge.
As the metal oxidizes it gives off a sweet taste in the horses mouth, this encourages salivation and acceptance of the bit, note all sweet iron bits as they oxidize will give the mouth piece the look of rust. 
Mouth size 4 1/2" has 65mm flat eggbutts and size 5" upwards have 75mm flat eggbutts.