Stamp rallies were especially popular during Expo ‘70. While going around to the different pavilions of the world featured at the Expo and collecting stamps, it really felt like you were travelling around the world.
There were various stamp designs sold at the expo, and there was even one with all the stamps on it. Apparently, this was to help lessen the burden of everyone, since there was an overwhelming demand for stamps. There was also something that the Japanese people of the time were excitedly collecting along with the stamps: autographs from foreigners.
Back then, it was rare to see foreigners in Japan, so a lot of Japanese people asked foreigners for their autograph whenever they saw them. There was a news report about a hostess from one of the pavilions who complained about how her hand hurt at the end of the day from giving her autograph to what seemed like an endless lines.
Incidentally, the self-inking stamp became famous from the Expo as well.