This is a very common style of Western curb bit generally called a grazing bit. The one depicted is a pony sized bit. Grazing bits usually have mullen mouths or low ports. The shanks are fixed to the mouthpiece rather than hinged.
A bit commonly used by Western riders for showing and pleasure riding. It can be a very harsh bit in inexperienced hands because of the long shanks.
How it Works:
The mechanics of this bit are that of a basic curb bit. When the rider places contact on the reins pressure occurs in the mouth, on the jaw through the curb chain or strap, and over the poll. The slight port on this bit provides room for the tongue.
From the collection of Rose Danko, Coates Creek Stables