Yes, as you say that is Oberbannführer Walter Seidemann. He was a Hauptabteilungsleiter within the Office for Sport (Amt für Leibesübungen) of the RJF (Amtschef was Obergebietsführer Ernst Schlunder). Seidemann appears to have held his position continuously between 1939 and 1945 although towards the end he probably had more pressing matters to attend to than sport
Are the other sports documents also from Weißflug?