Rastinirv sent me a PM asking me to examine his find.
I have to say, I've never seen anything like it before. It is an interesting mystery. Here are my observations and fianl deduction about it.
The milk-white patina indicates it is made of "pure-lead" -- not a hardened-lead alloy. As you know, pure lead is a very soft metal. That is why screws are never made of pure-lead.
Also, as you mention, it appears to have had a second "screw," which is broken off. The second "screw" is at a different angle from the other, which means that the object itself could not be screwed into something.
Therefore, as BosnMate already figured out, the only possible answer is that somebody LONG AGO poured molten lead into a notch in wood which contained two empty screw-holes. Then, over a period of many decades, the wood rotted away.
Supporting that deduction, note that the "screw" on your find has what is called wood-screw threading. (Screws made for use with a metal object have fine threading (usually 12-threads-per-inch, or more), and for use with wooden objects a screw's threading is coarse (usually 10-threads-per-inch, or less) -- like we see on your find.