The seals of religious brotherhoods are particularly rare, and our knowledge of these brotherhoods is extremely incomplete. Illustrating the scene of the Transfiguration, the recently rediscovered brass sigillary matrix of the Confrérie de la Transfiguration du Mont Saint-Aubert is examined here from several perspectives: typological, metallographic, historical, and iconographic. These make it possible to date its execution to between 1490 and 1510, and to link it to a group of works characterised by a "physical" representation of God, which is very rare in Transfiguration imagery. This matrix can thus be placed in the tradition of the artists of the Ghent-Bruges school in general, and of Alexander and Simon Bening in particular. More broadly, this study provides new insights into this little-known brotherhood, which was founded at a time when the Reformation was taking hold, and which probably developed sympathies for the Hussite movement.