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The question of Vatican participation in a stamp exhibition has been the ongoing question of the month. (see email discussions from <a href='https://vaticanstamps.org/cms/vslview.php?id=377'>February 5th</a> and <a href='https://vaticanstamps.org/cms/vslview.php?id=378'>February 6th</a>). So far, the debate has been between the Vatican Notes article by Joseph Lo Preiato from 1977 which stated that San Marino '77 was the first stamp show that the Vatican Post Office "participated" in and if the Vatican Post Office "participated" in the 1954 Marian Year Exhibition in Philadelphia. Well, new information has come to light! 
I have recently obtained a copy of the 1954 Marian Year Exhibition, published by the National Philatelic Museum (cited below). One of the articles in that booklet describes the origins of the Vatican Stamp collection and Vatican "participation" in the 1934 International Philatelic Exhibition in Milan: 
<blockquote>
"The collection was established and supported by Pope Pius XI. It came into being during the International Exhibition of Milan in 1934, when the Vatican exhibited a precious display of Roman States issues in its Pavilion together with 1929-1932 Vatican sets."
</blockquote>

To me, this sounds like the Vatican "participated" in the 1934 exhibition in Milan. Perhaps the "Vatican Post Office" was not the participant, but the "Vatican" participated. As far as the Vatican "participating" in the 1954 Marian Exhibit, I think not. The Marian Exhibit was a Cardinal Spellman event, perhaps with some co-operation from the Vatican, but not active "participation".

Any other thoughts?

Reference 
• Prof. Leone Gessi, "The Vatican's Collection" National Philatelic Museum Volume 7 Number 1 pages 148-153 (This publication was produced for the 1954 Marian Year Philatelic Exhibition held in Philadelphia September 22 - November 5, 1954.)

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