This small folding metal tool is a compact can opener developed by the US military for use in World War II.
It is called a P-38, but the origin of the name is unknown. It was likely a combination of the tool being 38 mm long and the popular military aircraft the P-38 Lightning that was introduced the previous year. It also supposedly took 38 punctures to open the C-ration can that the tool often accompanied.
To use, extend the blade so its perpendicular to the handle. Attach the notch in the handle on the ledge of the can. Using this ledge as a fulcrum, twist the handle so the blade pierces into the top of the can.
This simple machine is perfect addition to emergency or survival kits. It can also be stowed easily on a keychain. Many first-hand accounts boast about decades of multipurpose uses including screwdriver, boot cleaner, box opener, fingernail cleaner, and more.