Body lead.
anywhere from 5-10% to 30% Tin.
they also come in round bars of various diameters.
airc the size indicated the Tin content.
the higher or lower Tin contents were used on flatter or more vertical surfaces since you could use more or less heat to melt the lead and work it into place smoother without it running onto the floor.
now the sucky part,,,, I can't remember if bigger was more tin or less.
I'd bend it and listen to the creaking and popping if I heard it pretty well I'd call it 30% if just barely 10%.