Monaco issued a set of 4 stamps celebrating the 25th anniversary of UNESCO in 1971. Other stamps in the set featured: "culture" (scholar, book, film, television); "science" (alchemist, radar, rocketry); and the high value was an image of Prince Pierre of Monaco. The low-value in the set was called "arts" and included an organ case/facade and a detail of Michaelangelo's painting "The creation of Adam" on the ceiling of the Sistine Chapel in Vatican City, Rome, Italy. The case is distinctive, but one cannot determine the organ from the stamp alone. The Cathedral in Monaco has recently gotten a new organ. The stamp does not depict that instrument. Monaco has recently issued a stamp celebrating the new cathedral organ. When I have a copy, I'll post a new blog entry with an image of the stamp and (hopefully) a specification.
I heard from my friend Mark Jameson regarding the organ(s) in the Monaco cathedral:
I have found the 1934 spec of the 1922 Charles Mutin organ that lasted until 1968, original organ was c1887. In 1971 I suspect the Mutin had been replaced by an Allen. I have e-mailed Olivier Vernet [organist] to try to obtain more history for use in the next Journal – Monaco & organ, San M, Vatican & Italy for stamps.