During the Vietnam War the U.S. Army established fire bases in various locales to support the far flung forward operating /patrol bases set up to...well...patrol the country side. The fire bases were locations where emplaced artillery, either towed or self-propelled, could rain artillery fire down on unsuspecting enemy combatants and the occasional, errant water buffalo.
These fire bases were more often than not semi-permanent operating bases. With the passing of time the personnel at these areas would embellish and improve their living and work environments. Improving that work environment often took the form of some creative DIY projects. One such improvement was the stowage of ammunition for the 'Tube'. [ Technical Note: 'Tube' is cool artillery talk for the actual artillery piece.]