40mm Mk III Shell Casing 1955
This 40mm Mk III anti-aircraft shell casing was typically fired from a single or double barrel Bofors 40 mm L/60 Model 1936 artillery gun. Typically, the double barrel version was produced for the navy and a single barrel version for the army. This particular shell casing is stamped with a 5 - 55, indicating that it was produced in May of 1955. There is also a small impression of an eagle with 3 stars above it. That symbol is the Department of Defense acceptance insignia.
For demonstration; see the photograph of a double barrel cannon firing aboard the USS Hornet.