An old steel ring bit.
This severe mouthpiece with its high port and ring that acts as a curb is also known as a "Mameluke" bit.
This example is handforged in iron or steel which has acquired a rich brown patina.
Its short curve cheeks would originally have been decorated with silver studs 

The bit has a high port with wires across, one copper roller remains in the port, there are also copper rollers on the mouthpiece
The ring has cast and engraved decorations

Dimensions of bit : Mouthpiece 10cm (4")
Cheeks 6.5cm (2½")
Overall length 23cm (9")
Weight 244g (8½ oz)