Your find is definitely the discarded leftover lead from the bottom of a lead-melting pot.
Long ago, I was a plumber's helper... and also, I cast my own lead bullets for my repro civil war revolver. In both of those jobs, I used a lead-melting pot, and I'm familiar with what the last bit of lead in the bottom of the pot looks like. It is typically discarded because the last dregs tend to contain impurities and tiny bits of dirt.