Please reread Dorleac's posts: "... or perhaps his beloved .300 Flanged. He used it quite a lot on wild boars...", "... the photo was taken at Vauguion...", earlier: "He owned other H&H Royal double rifles, but didn't show much interest for these guns except for a Super 30 that he especially liked and that I saw him use regularly for driven hunts at Vauguion."
IMHO it is very likely that he holds his H&H .300 Flanged aka Super .30 in the trophy photo, as that's the rifle he used regularly when he was in the place where the photo was taken, Vauguion.