The Century of Progress Exposition: A Century of Progress International Exposition held in Chicago in 1933 was a major philatelic event of the century. Several U.S. Stamps were issued, but the real story of the Century of Progress Expo are the First Day of Issue Covers and the many Zeppelin Covers and flight covers of interest to stamp collectors.

The Postal Progress Theme: The cachet in the image shows transportation evolution from 1834-1934:

• A horse-drawn stagecoach (representing 1834 mail delivery)
• A steam locomotive (mid-19th century rail mail)
• An airplane (modern 1930s airmail service)

This illustrates the dramatic transformation in how mail was transported over the century, from slow overland coaches to fast railroad service to rapid airmail delivery.

The San Pedro Connection: The cachet indicates it was "Sponsored by San Pedro P.O. Employees" during "Calif. Spanish Fiesta and Exposition Week," suggesting this was a local postal celebration tied to the broader Century of Progress theme, commemorating how mail delivery had advanced from frontier days to the modern era.

Philatelic Significance: The stamps were reissued on August 25 in imperforate souvenir-sheet form without gum to mark the convention of the American Philatelic Society. The exposition generated significant philatelic interest and numerous commemorative covers celebrating both technological progress and postal service evolution.

This cachet celebrates the remarkable transformation of American mail service over 100 years, from horseback and stagecoach delivery to modern railroad and aviation networks.