This bit was originally called the Kimblewick after the English town where it first appeared. I believe it was invented and designed by Phil Oliver, the father of international Show Jumper and course designer, Alan Oliver.
However, in its early years of use, it was also known as the "Spanish jumping bit." The "D" ring is offset, so the bit mouthpiece is on the upper part of the flat side of the D, creating a small amount of leverage, supported by a curb chain. This allows the Kimblewick to have a mild curb bit effect.
Like the pelham and curb bits in general, the Kimblewick has bit shanks with purchase arms. However, unlike these other bits, its shanks have no lever arm. Due to the purchase arm and geometry of the rings, the rings may function as very short lever arms and create a small amount of leverage, which puts this type of bit into the pelham or curb bit "family."