This particular bit is a fixed-side, low, or shallow-ported curb bit, such as is used, along with a bridoon (snaffle) bit in a double bridle.
Ported bits, including pelhams, kimberwickes and double bridles, have ports of varying degrees.
This shallow port allows the mouthpiece to act on the lower jaw with pressure on the tongue, rather than on the bars of the mouth.
A high ported bit allows more room for the tongue and the bearing surface of the mouthpiece acts more on the bars of the mouth.
The curb bit, by its action on the lower jaw, on the chin groove and across the poll, causes the horse to lower his head, the nose brought into a more perpendicular position and the lower jaw to relax.