This is an old steel Hanoverian horse bit with quite a dark patina.
I wrote a short blog about Hanoverian bits here
It has a medium height port with three rollers on each side of the port.
The rollers still turn freely.
There is a hinge on one side of the port but not the other - this is unusual, perhaps the bit was a special commission.
It has curved cheeks or shanks which turn.
The original curb hooks survive.

Dimensions : Mouthpiece 5" (12.8cm), total cheek length ( not including curb rein rings) 8" (20cm)
Weight : 514g (1lb 2oz)