Since 1958, all Zippo lighters have a date code stamped on the bottom. This code has had many iterations over the years. in chronological order, the date was indicated by dots, vertical lines, slashes (both directions), roman numerals and today’s Arabic numerals throughout the years. Slim Zippo’s had different codes than the regular Zippo’s between 1956 and 1965. Look at the bottom of your lighter and choose between de links, or scroll to see all date codes :).