Robert Lincoln would not allow his father’s papers to be opened until 21 years after his own death, although he did allow his father’s secretaries earlier access.
Robert died at his 32-room summer mansion in Vermont on July 26, 1926. Exactly 21 years later, at the stroke of midnight, the papers were opened.
Among those present for the opening were U.S. Grant III, grandson of President Grant, and Robert Todd Lincoln Beckwith, great-grandson of President Lincoln.
Source: Ralph G. Newman in the Bulletin of the 28th Annual Meeting of The Lincoln Fellowship of Wisconsin 1981