"Verisimilitude" is defined as...

...the appearance or semblance of truth; likelihood; probability

The story's verisimilitude does not come from diction or plot, for much of the story is like a shopping list: a catalog of the many things the soldiers out in the fields of Vietnam carried. Many of the supplies involved a thorough collection of weapons and provisions for the battlefield. Others, the items that provide more insight into the individuality of the characters, are found in the personal possessions each carries to enable him to stay connected to family and friends at home—connected to the normalcy of a former life as they face the possibility of death each day.

The literary device that makes the belongings so impactful is the use of the general device of imagery. Imagery is defined as...

...the forming of mental images, figures, or likenesses of things.

Imagery creates vivid mental pictures by appealing to the senses.

The characters become realistic—and memorable—as we visualize the personal effects they carry. The use of imagery supports the "stylistic feature" of realism that O'Brien strives to impart in his story—this conveys the men as real people rather than "one- dimensional" characters.

In the first week of April… Lieutenant Jimmy Cross received a good-luck charm from Martha. It was a simple pebble.

Jimmy Cross carries the pebble because it connects him to Martha, a girl he believes he loves, at least for a short time. That connection brings to his mind pictures of the beach from which she took it.

Kiowa carries a book that might feed his soul, but which he also used for physical comfort—a typically human response:

Shrugging, Kiowa pulled off his boots. He wanted to say more, just to lighten up his sleep, but instead he opened his New Testament and arranged it beneath his head as a pillow.

The other things they carried included their hopes and their intense fears...

For the most part they carried themselves with poise, a kind of dignity. Now and then, however, there were times of panic...When they twitched and made moaning sounds and covered their heads and said Dear Jesus and flopped around on the earth and fired their weapons blindly and cringed and sobbed and begged for the noise to stop and went wild and made stupid promises to themselves and to God and to their mothers and fathers, hoping not to die. 

While some of these things were observable, the emotional baggage they carried allows us to see how human the men were, and how difficult their tour in Vietnam was.

They carried all the emotional baggage of men who might die. Grief, terror, love, longing—these were intangibles, but the intangibles...had tangible weight. They carried shameful memories. They carried the common secret of cowardice barely restrained, the instinct to run or freeze or hide, and in many respects this was the heaviest burden of all, for it could never be put down...

For many people who experienced the war via video footage on the six o'clock news or from reading the news paper or history books, O'Brien's imagery "welcomes" the reader into the world he saw—the one in which he lived and survived—making real the horrors of war fought in an unfamiliar land, fighting enemies about which they knew so little.

Imagery is the most impactful literary resource that O'Brien uses in his story.