This 1958 QSL card was created by Dr. Henry Richter, one of the key engineers in the Explorer 1 mission. W6VZA was his callsign. He distributed a limited number of these cards in celebration of the successful launch of Explorer 1.
Many thanks to Jeff Olivares, N6ONE, for providing scans of this QSL card! He was given the card by Dr. Richter at one of the talks he gave to the JPL Amateur Radio Club.
For more information about the Explorer 1 Mission, visit the JPL Explorer 1 Mission web page.