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", 4 1/2" & 5" have ring sizes of 55mm and sizes 5 1/2" & 6" are 65mm.