One Finger Knuckles / knuckle-duster

Brass knuckles are "fist-load weapons" used in hand-to-hand combat. Brass knuckles are pieces of metal shaped to fit around the knuckles. Despite their name, they are often made from other metals, plastics or carbon fibers. Designed to preserve and concentrate a punch's force by directing it toward a harder and smaller contact area, they result in increased tissue disruption, including an increased likelihood of fracturing the intended target bones on impact. The extended and rounded palm grip also spreads across the attacker's palm the counter-force that would otherwise be absorbed primarily by the attacker's fingers, reducing the likelihood of damage to the attacker's fingers.

This one have special hole for string to wear on neck