P-51D Mustang Letter Opener is comprised of two original parts that would have been found in an original stock P-51D Mustang.
The stand is the Arm Assembly (Fig 95-3 in the parts catalog) that is contained within the Fixed Gun Rear Mount Assembly (Fig 95 in the parts catalog). This unit was part of the Wing Assembly and was used to hold and adjust the rear end of the six 50 caliber guns in the wings.
The letter opener itself is the Dipstick (Fig 56-24 in the parts catalog) and is used to check the hydraulic fluid level. It has been modified by grinding a radius on one side to create the slightly sharpened edge.