I have a partial idea. When casting lead (plumbers and shooters) a cast iron pot is/was used. The unused lead cools and takes the shape of the pot. And lead does not stick to cast iron or steel/stainless steel (handy to know).
But someone would have had to cut a slice off the piece (easy enough - I have chopped up lead with an ax & hatchet and even a hand-plane). The hollow is a little more difficult.
Possibly something was tossed in as the lead was cooling or it was dumped out before fully cooled and rested on an earlier pot molding(

) Or someone left the drossing spoon on top as it cooled.
Molten lead is fun to play with - steel floats on it.