It's a question that drives the extraordinary German site Lost Films. Begun in December 2008 by the Berlin museum Deutsche Kinemathek, it's a collaborative effort with other archives that now encompasses an astonishing range of films: The more than 4,000 movies listed as M.I.A. range from an actual jazz-era version of The Great Gatsby (1926) to a re-enactment of The Battle of Gettysburg (1913) staged while the veterans were still alive. But even more curious is the site's "Identify" section—an open call to other museums and the public to I.D. films that sometimes survive without title cards, without canister labels, without so much as a cast or director or country of origin....