There are 38 manuscript letters signed and two typescript pieces of correspondence. Of the 36 papal letters the oldest is on vellum from Pope Alexander IV, dated 1256; the most recent typewritten one is from Pope Paul VI, signed G.B. Montini on April 6, 1938. The four remaining letters are from St. Francis de Sales, St. Gregory Barbarigo, St. John Leonardi, and St. Vincent Maria Strambi.
The entire collection can be viewed online here.