Here’s an old U.S. Civil War relic at the Varnum Memorial Armory Museum showing evidence of having been left outside for several years. It was not uncommon for people roaming America’s battlefields in the years and decades following 1865 to find souvenirs like this.
This is a Model 1816 Springfield musket converted from flint to percussion. It was a common firearm in the first half of the American Civil War. The stock inspector cartouche is still visible.