Pardon me, please, but I have to disagree that it is a lead ingot/pig.
I believe it is simpy the "leftover" lead from the bottom of a lead-melting crucible (pot). I've "made" some of those myself, when casting bullets for my Repro civil war pistol. Also, I've been a Plumber's Helper, and seen what happens to the leftover lead in the crucible after work is finished. Particularly when there's only a small amount left, it tends to contain slag/impurities, and thus gets discarded.
I should mention that I've also dug a couple of those lead crucible leftovers in the yards of 1800s-to-early-1900s houses in downtown Richmond, where lead was used to seal the joints in large cast-iron plumbing pipes.
Regards,
Pete