In perhaps the largest mail drop in art history, Leonard A. Lauder gave 20,000 postcards to the Museum of Fine Arts, Boston in 2002. "Art of the Japanese Postcard" displays 350 of them, dating from 1900 to 1940. Two things are striking about them other than their range, wit and beauty: how slyly self-referential many of them are and how many ways they find to be self-referential. But first, some history.