I have acquired another Musée du Luxembourg postcard of Mater Triumphalis, undated but presumably 1915-1922, the period when the painting was housed at that location. It is in better condition than the other postcard and has a better-quality photograph, so is perhaps later in that period. It shows hints of the discolouration appearing on the hip and legs that the painting currently bears, so this appears to have started developing c. 1920.
The painting is called Mater Triomphalis on these cards (a typo caused by confusion with the French word for triumph, ‘triomphe‘), but the correct title is Triumphalis.
The two postcards (c. 1915 and c. 1920?) and the painting today compared.