Answer: Cardinal Francis Spellman (1858–1957) was described as "a very enthusiastic, discriminating collector and a serious philatelist at heart." The article referenced below describes the uniqueness of Cardinal Spellman's sheet of the Vatican City Roman States Centennial stamps:

"His Eminence revealed that he had expressed a desire to have a stamp autographed by the Pontiff on several occasions when he was in Rome, but not until 1952 did he have his wish fulfilled. In sending it to him, His Holiness stated that he never before had autographed a stamp, and never would again so this single item will be philatelically unique."

Note that the Septermber / October 1959 issue Vatican Notes lists Cardinal Spellman as a member of the Victican Philatelic Society (Membership Number 1000).

References
• Ernest A Kehr (editor) "Cardinal Spellman, Philatelist" National Philatelic Museum Volume VII Number 1 Pages 37 & 88

For More On Cardinal Spellman see:
• Francis Spellman Wikipedia