“[O]n June 4, a laborer working on construction of the new American Wing at the Museum of Fine Arts knocked a hole in a wall and saw an envelope sticking out of the rubble.” It was “a typewritten note from 1926, a letter to the future from a long-ago laborer who helped build the wall.” A historian who directs the museum’s libraries and archives “has enlisted the help of history buffs, genealogists, and archivists – as well as the power of the Internet – to piece together his story.”