This is a very old iron tsuba (sword guard). It has an arm-hole (腕貫孔) but no kozuka-hitsu (knife hole) or kogai-hitsu (hairpin hole), and is a perfectly round tsuba, likely made by a swordsmith. It's a rather large tsuba, probably made for a large sword. It's well-forged and has a beautiful, deep black iron texture. I believe it's a very old piece. 8.5 cm × 8.5 cm × 3.8 mm