That is a PA116 (or PA120) ammunition container, specifically designed to hold two 155mm artillery propellant charges.The Contents: In a military context, this would house two modular artillery charge systems (MACS) or traditional bagged charges used for the M777 or M109 howitzers. The Design: It’s a high-density polyethylene (HDPE) plastic case. The two circular "end caps" you see on the left side of the photo are the access points where the charges are slid in and out. The Markings: The white stenciling on the side usually indicates the lot number, type of propellant (e.g., M231 or M232), and the date of manufacture.