The Project Mercury stamp was released for sale upon the successful splashdown (on Feb 20, 1962 at 2:43PM) and recovery (at 3:01PM) of the Friendship 7 spacecraft that carried John Glenn in it. At 3:30PM on that day, 305 postoffices all over USA released the Project Mercury stamp for sale to the public. This stamp was designed by Charles R. Chickering, printed in secret and distributed to the post offices. Even these post offices did not know what they had receive until word was given to open the packages and put the stamps on sale. Since this was a "Top Secret" stamp issue, many post offices only had the stamps on sale for a couple of hours.
Project Mercury stamp block signed by the stamp designer, Charles Chickering.
Article: Charles Chickering, designer of the Project Mercury stamp.