Definitely the discarded leftover lead "pudlle" from the rounded bottom of a lead-melting crucible. Most frequent source (but not nearly the only one) are oldtime plumbers, who used the molten lead as poured seals around cast-iron pipe joints. I'm sure of the ID for two reasons:
1- I used to be a plumber's-helper.
2- I've made cast-lead bullets by melting scrap lead in a rounded-bottom crucible. The leftover lead puddle has the same shape as your finds. The rippling/crevices is due to the impurities which are often present in scrap lead.