NEW YORK — For many kids of the postwar generation, the best part of back-to-school shopping was not the clothes or the annual trip to Buster Brown to get a new pair of shoes, but choosing a lunchbox. Brown-bagging your PB&J sandwich became a thing of the past with the advent of bright and colorful metal lunchboxes decorated with child-friendly characters or favorite media stars. In school cafeterias from as early as the 1930s through the early 1980s, a parade of sturdy lunchboxes featured fictional and real-world stars such as Mickey Mouse, Batman, Strawberry Shortcake, Superman, the Beatles and Scooby Doo.