The Seventh World Jamboree was held in Austria in 1951, with a special commemorative issue.
Later in the year after the Jamboree closed the stamps were used in connection with a balloon flight to the Children s camps on the Balloon Zurich on October 10, 1951.
Mail going through the Soviet occupied zone had to be censored and bear the markings and tape re-sealing of this mail where cards and letters from the free world passed through the Iron Curtain. Note the inspection mark in the upper left corner.
SOSSI Journal, Volume 44, Number 4, July-August 1995