Now that's a pretty cool card, right?  If you're a "ham" (a.k.a. amateur radio operator), you know exactly what this is.  For those unfamiliar, it's a traditional way to prove you've had radio contact with another ham.  Before the Internet, these cards were the only way to establish proof, and hams would exchange these via snail-mail.  They were useful for ham contests (of which there are still many) to help rack up points for recognition and rewards.  This is officially called a "QSL card."