Wondrous Item, rare (requires attunement)
These heavy brass knuckles were custom made for the six-fingered monk Callianne Arleberry. Engraved on each knuckle are letters spelling: M-O-N-K P-U-N-C-H. Attuning to these knuckles causes a creature to grow an extra finger on their left hand, and they become left-handed if they aren't already. In addition, they gain +2 damage to unarmed strikes and they can use their reaction to deflect weapon attacks gaining +2 to their AC against the triggering attack.
Notes: Damage: Unarmed Attacks, Damage, Combat, Handwear
Maybe consider using a Ki point or two to activate the +2 AC bonus? Seems a little overpowered to allow it every turn. Should probably also be restricted to melee attacks, particular because Deflect Missile Attacks is a standard Monk power.