It was 1874, a challenging year for Susan B. Anthony and the women’s suffrage movement. She had been convicted of illegally voting in Rochester, her hometown, and activists were split over the best way to push the movement forward.

On April 9, Anthony wrote to a fellow activist named Isabella Beecher Hooker, looking ahead to better days.

“Now wouldn’t it be splendid for us to be free & equal citizens, with the power of the ballot to back our hearts, heads & hands,” Anthony wrote, envisioning a time when women could also fight for “the poor, the insane, the criminal,” armed not just with moral suasion but “with power too.”

“I can hardly wait,” she continued. “The good fates though are working together to bring us into this freedom.”

It would be 46 more years before the 19th Amendment guaranteed women the right to vote. The letter, saved by Hooker, ended up stashed in a wooden box along with nearly a hundred others relating to the suffrage struggle. And there they sat for more than a century, unknown to historians or seemingly anyone else.

Now, the letters have been acquired by the University of Rochester as part of a larger trove of material that some are calling the biggest discovery of its kind in decades.

“It’s a stunning collection,” said Ann D. Gordon, a retired professor at Rutgers University and the editor of the Elizabeth Cady Stanton and Susan B. Anthony Papers Project. “That it’s being delivered all in one piece, with such a clear provenance, is remarkable.”

The collection, which Ms. Gordon is so far the only outside scholar to see, includes 26 letters from Anthony, 10 from Stanton and dozens from other suffragists. There are also broadsides, pamphlets, newspaper clippings and other material that Hooker kept in a kind of circulating library, many with “I.B. Hooker, please return” marked in her handwriting.

The material, mainly dating from 1869 to 1880, may not upend current scholarship, but Ms. Gordon said it sheds light on a contentious period within the suffrage movement, while underscoring the degree to which the movement was driven by complex networks of on-the-ground activists.

“We don’t pay enough attention to what a local movement this was,” Ms. Gordon said. “We’ve warped the story by only knowing the names of the national leaders.”

Isabella Beecher Hooker, the youngest daughter of the prominent minister Lyman Beecher, had lived with her husband and three children for a time in Nook Farm, a literary colony in Hartford, whose residents included her half sister, Harriet Beecher Stowe, and Mark Twain.

The collection came to light last year, when Elizabeth and George Merrow of Bloomfield, Conn., were cleaning out a jumble of books and papers, including some that had been retrieved from the former Hooker home, which Mr. Merrow’s grandfather, heir to a major sewing machine manufacturer, bought in 1895.

Bob Seymour, a local book dealer, had recognized the signature of Elizabeth Cady Stanton on one letter. He sent the box to a colleague, Adrienne Horowitz Kitts, who didn’t initially have high hopes.

“We figured we’d get some stuff about the family, maybe some New England manufacturing history,” she said, referring to the Merrows.

But as she started sorting through the material, dusting away ample mouse droppings, she was stunned to realize it was a rich archive of suffrage material.

“It really shows you what these women went through,” Ms. Kitts said. “They really busted their butts for us.”

Lori Birrell, the special collections librarian for historical manuscripts at the University of Rochester library, said she was moved by a letter from 1872 in which Hooker describes her feelings of inadequacy in an early political speech.

“Sitting here in 2017, that really blew me away,” Ms. Birrell said. (The library, a leading repository of early suffrage material, did not disclose the price it paid for the collection.)

But Ms. Gordon said the collection was very much “a political archive,” illuminating the years immediately following 1869, when the suffrage movement had split over the 15th Amendment.

A New York faction, led by Anthony and Stanton, opposed the amendment on the grounds that it would enfranchise black men but leave out women of all races. A Boston-based faction, led by Lucy Stone and others, strongly supported it, arguing that including women would doom the amendment and that women’s suffrage would be better pursued at the state and local level.

“I believe that just so far as we withhold or deny a human right to any human being, we establish a basis for the denial and withholding of our own rights,” Stone wrote to Hooker on Aug. 4, 1869, in a letter in the new collection explaining her support for helping black men win the vote first.

There was a fight about which side would get Hooker, who was “considered an important catch,” Ms. Gordon said.

Ultimately, Hooker went with the Anthony-Stanton wing. But she still worked to unify the movement. In 1871, she organized a national suffrage convention in Washington, which pushed the argument that women, under the recently adopted 14th Amendment, were citizens and therefore already had the right to vote.

The letters, Ms. Gordon said, also depict Hooker’s tireless work on the ground in Connecticut — one of at least 22 states where women attempted to vote between 1868 and 1873.

Hooker died in 1907, 13 years before the 19th Amendment went into effect. She was important, Ms. Gordon said, not just as an activist but also as an archivist.

“She was someone who was part of the conversation and decision-making, and also someone who kept everything,” she said. “That’s huge.”