FDC (First Day Cover) stamps were issued on March 5, 1973, commemorating the Consistory in which Pope Paul VI named Albino Luciani, the future Pope John Paul I, Cardinal.
He was one of seven men so named in 1973, and he is pictured at the far left.
FDC is a term collectors use to designate a postage stamp on a cover, postal card or other stamped envelopes on the first day that the issue is authorized for use within the country or territory of the stamp-issuing authority.
The Vatican, of course, because it is a sovereign nation, produces its own stamps, and it is one of the most profitable sources of income for the Vatican due to the high demand for them by tourists.
This is a very nice item depicting the newly made Cardinal Albino Luciani.