On September 28, they ascended the hill Cumorah in New York and reported the experience in an article in the Deseret News.
Sitting on the hill Cumorah, Stevenson wrote that "it was here, more than twenty-four centuries ago that the descendants of Jared and his brother fought their last exterminating battle... It was also in this land that Mormon in his old age and his son Moroni led their Nephite armies against their brethren the Lamanites... This took place more than fourteen hundred years ago, but looking over this hilly country today... our imagination can easily conceive how the exile Moroni... emerged from his hiding place and deposited the sacred treasure in this hill..."
Stevenson knew Oliver Cowdery, David Whitmer, and Martin Harris. His bio explains,
He said of Oliver Cowdery: "I have often heard him bear a faithful testimony to the restoration of the gospel by the visitation of an angel, in whose presence he stood in company with the Prophet Joseph Smith and David Whitmer. He testified that he beheld the plates, the leaves being turned over by the angel, whose voice he heard, and that they were commanded as witnesses to bear a faithful testimony to the world of the vision that they were favored to behold, and that the translation from the plates of the Book of Mormon was accepted of the Lord, and that it would go forth to the world, and that no power on the earth should stop its progress."
Elder Stevenson said of David Whitmer, in 1886, after recounting previous testimonies he had listened to from him: 'David Whitmer is now just past eighty-one years of age. In this his last testimony he said to me: 'As sure as the sun shines and I live, just so sure did the angel appear to me and Joseph Smith, and I heard his voice, and did see the angel standing before us, and on a table were the plates, the sword of Laban and the ball or compass.'"
Concerning Martin Harris, Elder Stevenson wrote: "In the year 1869, I was appointed on a mission to the United States. Having visited several of the Eastern States, I called at Kirtland, Ohio, to see the first Temple that was built by our people in this generation. While there I met Martin Harris, soon after coming out of the Temple. He took from under his arm a copy of the Book of Mormon, the first edition, I believe, and bore a faithful testimony just the same as I heard him bear thirty-six years previous.
- Aug. 19, 1870, in company with Martin Harris, I left Kirtland for Utah, and on the 21st he was with me in Chicago, and at the American Hotel bore testimony to a large number of people of the visitation of an angel, etc."
Stevenson clipped the article and put it in his scrapbook, which you can see here:
1888 article (click to enlarge):