Blue sweet iron mouth on stainless steel.
 
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. 
 
Size 4 1/2" has 60mm rings, size 5" and upwards has 65mm rings. 
 
Ported mullen mouth leaves space for the tongue, relieving pressure.