Brass Knuckles is a common Artifact — Equipment card from Commander Masters (CMM), collector number 372. Current market price: $0.22 (foil $0.40), TCGplayer market prices via Scryfall.
When you cast this spell, copy it. (The copy becomes a token.) Equipped creature has double strike as long as two or more Equipment are attached to it. Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)