Answer: The Vatican Postal Administration participated for the first time in a philatelic exhibition at San Marino (San Marino 77) which ran from August 28 to September 4, 1977.


The cancellation reads: Philatelic Exposition San Marino 77 with the show dates. This cancelation was used on covers from the Vatican as well as on special souvenir folders handed out at the Vatican booth at the show.

See:
Joseph Lo Preiato, A First... By The Vatican Post Office Vatican Notes Volume 26, Issue 3, 1977, Page 11