It looks like part of a lead bar. It could have been used for a lot of things. I don't know if you still can but you used to be able to buy one poud bars of lead for plumbing, autobody work, and a lot more.
Lead was used for so many things and came in so many shapes and sizes over the years, this could be most anything. It looks like a piece cut from a small bar of lead most like likely used for melting to pour lead in cast iron pipe joints.