The creation of the Universal Postal Union spurred a competition among those countries that thought their postage stamps could be more beautiful, and, in fact, actually do more than those of other nations.
This Haitian 5-centime measured the p.s.i. inside an envelope to warn recipients of contents packed under pressure.
Another cautionary issue, the Netherlands’ 5-gulden stamp indicated the voltage of a letter, using the classic Norton Voltmeter.
The New Zealand 4-pence included a working compass for those traveling with the mail.
The Malta 2-pence “Silvertone” brought in radio stations from all over the world.
The short-lived U.S. 10-cent “Safe Lock” stamp offered remarkable security, but unfortunately senders often included the necessary combination inside the letter, thus locking out the recipient who had to visit either the Post Office or a locksmith to gain access.