I'm trying to understand your cover, Jamesw. It looks like the 1939 stamp and cancel were added to an earlier cover and cancelled as a favor item. Is that right? Do you think it did actual postal service in 1939?
I forgot about this thread, but I do like the topic. Here's a reprint the BEP did for the centennial of the Jamestown Exposition. The original souvenir card from 1907 was in black ink and did not include the stamps at the bottom.
Let's get this thread back on its feet! I happened on this cover from Expo '74 in Spokane. I think this is actually non-philatelic. It appears to have been mailed at the fair, so apparently there was a post office on site.
Looking through this thread I saw a couple of World's Columbian Exposition cards. Here are a few more. Unfortunately someone trimmed the last one, probably to fit in an album. All are the same on the address side.
Here's an unmailed cover that seems appropriate for this thread. It appears to be a SASE for the Alaska-Yukon-Pacific Exposition, held in 1909 in Seattle. Apparently the fair headquarters were actually onsite in the Alaska Building.