I have seen these used to seam joints in copper gutters and down spouts.
Years ago, I was working for a roofing company.My company was hired to replace the slate roof and gutters/down spouts on the Admin. building (Golden Dome) of the University of Notre Dame.
Sheetmetal workers(tin knockers) would have a propane heater pot.In the pot were like smelting tools,which would be glowing a red hot in color.
Tool in one hand and lead rod in the other.Putting the two together, melted the lead to the seams.
I was a carpenter on this project replacing the wood that formed the gutters with copper plate overlay.