Hmmm, well,... I don't think it's a stirrup, it looks like a bit covered in leather w/the noseband attached. Maybe the rings at the bottom are for a curb? Maybe it's some crazy kind of bit guard.
It is a bit.
It kind of works like a Kineton
2 reins- 1 to the bit the other to the o-rings on the leather strap.
When you pull back, it applies pressure to the mouth and the face.
Used on heavy pullers.
Great wall decoration.