US WWII-era 37mm Projectile w/ casing

Identification

From the base markings and form:

• Caliber: 37mm
• Type: Likely from a 37mm M16 / M3 anti-tank or aircraft cannon round used by the U.S. during World War II.
• Date: Your base stamp appears to show 1942.
• Maker mark: The “P.D.” marking commonly refers to Plymouth Ordnance Plant (or similar wartime production facility) depending on the exact stamp variant.
• Construction:
  
• Brass cartridge casing
• Steel projectile with copper driving band
• Primer still visible in the base (inert)

These rounds were commonly saved by soldiers and turned into decorative trench art souvenirs after being fired or recovered. Trench art refers to decorative objects made from battlefield materials such as shell casings, bullets, and scrap metal during wartime. 

Your example is fairly typical:

• projectile placed back into the casing
• heavy patina from age
• minimal decorative modification

Condition

• Heavy oxidation on projectile
• Copper driving band visible
• Brass casing with natural patina
• No engraving or heavy modification

Because it lacks engraved decoration, it falls into the simple trench art / inert display round category rather than high-end soldier-engraved pieces.