Some people have guessed a hammer, but it seems much too small for that unless it was one of the seven dwarfs' hammers. Others have suggested a railroad spike, but it's much larger at the end than at the top, so I don't think so.
There was a lot of WWI fighting in our forest, and some WWII fighting as well, so our guess is something from one of the wars. Frederic's coworkers think it is a leftover piece of a bomb. We could clean it up so we could see it better, but I'd like to be assured it's not going to blow up in our faces before we attempt that.
What is your guess?