An old iron ring bit.
This style of bit with its severe mouthpiece with a 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 russet patina.
The bit has a high port with a bar across which holds two small rollers and a disc.

A ring hangs from the top of the port. 
Half of the a curved slobber or shackle bar remains

Dimensions of bit : Mouthpiece 10cm (4")
Cheeks 7.6cm (3")
Overall length 30cm (11¾")
Weight 384g (13¼ oz)