The Daily Question
Which church has the mosaic depicting the six-winged seraphim angel found on the airmail stamp (Scott #C59) that the Vatican Post Office issued in 1974?
1. St. Peter's Basilica in Vatican City
2. St. John Lateran in Rome
3. Basilica of the Nativity of Saint Mary in Milan
4. St. Mark Basilica in Venice
Your answer was: St. Mark Basilica in Venice
You are correct!!
Answer: The mosaic depicting the six-winged seraphim angel found on the airmail stamp (Scott #C59) that the Vatican Post Office issued in 1974 can be found in St. Mark's Basilica in Venice. The Byzantine-style six-winged seraphim angel is a common form of decoration associated with the Greek Orthodox Church. St. Mark's design has a lot of Greek Orthodox influence.
See also:
• Rev. H. A. Phinney,
A Brief Historical Sketch - St Mark's,
Vatican Notes, Volume: 22, Issue 5, 1974, Page: 8

Byzantine Angel Mosaic in
Basilica of St Mark in Venice
Scott # C59 (1974)