Wilhelm Brenneke experimented a lot with .404 base cases, read W.B.Dixon’s “European Sporting Cartridges”. About 1925 he designed the 9.3x70 for DWM. The 1934 DWM catalog only listed it with 18.5 g = 285 gr bullets at 845 m/s = 2770 fps, 17.0 g = 262 gr at 854 m/s = 2800 fps, 15 g = 231 gr at 905 m/s = 2969 fps. As that large case with the powders and primers then available produced unreliable ignition in cold weather, it was soon discontinued before WW2. Brenneke designed the 9.3x64 instead.
In 1937 Brenneke experimented with his 9.3x64 case necked down to his first 7x64.5 Brenneke Maximum. A year later he experimented again with the .404 case, now shortened to 66 mm and the rim slightly rebated. He created a 7x66, a 8x66 and a 9.3x66. The 8x66 Brenneke Maximum only was assigned a DWM case number #598. None of these experiments ever reached commercial status. So ballistics are unknown. Brenneke collaborated closely with DWM in developing his cartridges and loads. As Walter Gehmann worked for DWM at that time, he was certainly aware of Brenneke’s ideas and experiments.