Hi Andi, thank you i am quit happy with them as well
The surface is mostly smooth with no overlapping lead, so i would say they where integrally cast. They probably selected stones or pieces of stone that would fit within a mould and put those pieces in the mould before pouring the lead in.
There are also a couple of lead balls with an iron core, made in what i supose is the same way. The pieces of metal are not al round, but seem to have corners apart from the heavy oxidation.