IUPAC in Brussels in 1921: A Historical Photo

During a search of photographs and documentation on the Belgian photographer Benjamin Couprie, who took the well-known pictures of the Solvay Conferences during the first half of the twentieth century, Santiago Alvarez [1] came across an image in “La Digithèque des Bibliothèques de l’Université Libre de Bruxelles” with the title “Réception de l’Union Internationale de Chimie Pure et Appliquée, photographie de groupe” (Figure 1) [2]. It is a high-resolution copy of a very sharp photograph of a group of 86 people. On the frame of that photo one can read two inscriptions noted in pencil above and below the photograph, respectively: “Union Internationale de la Chimie pure et appliquée,” and “Reception au chateau de La Hulpe le 29 Juin 1921.” Moreover, just under the photograph there are two inscriptions in smaller letters: “Bruxelles 1913,“ on the left, and the signature of “Benj. Couprie” on the right, both in the same handwriting. Two questions arise: (1) Which is the correct date for that photograph? (2) Who are the persons that appear in the photo?