The rare Whitman “soldier letter,” one of only three known to exist, was discovered last month by a National Archives volunteer who is part of a team preparing Civil War widows’ pension files to be digitized and placed online.

“It doesn’t get much bigger, in my eyes,” said Jackie Budell, an archive specialist who oversees the project. “It’s just simply stunning. . . . We’re not going to find another one like this, probably, for a while.”

Whitman, the American poet, journalist and essayist, was known for making the rounds of the local hospitals, where he would dispense snacks and money. He would also sit with wounded and dying soldiers and write letters for them.