It doesn't appear to be American, those all seem to be much larger and more complex.
However, there's a couple of old Soviet designs that bear a striking similarity. One example- second photo from the bottom, middle sample- "Russian 82mm HE". Second example- middle photo a bit more than halfway down. Five sizes of shell that all appear to share a similar style fuse.
Chances are you don't have to worry about it, it's almost certainly from an expended shell. Or possibly from a round that was fired but didn't detonate, and the thin sheetmetal shell eventually rusted away.
If the latter, there's a nonzero chance there's still some small amount of priming compound in there, but even if so, chances are it's seen so much weathering most of the chemical is gone.