If the cartridge 404 Magnum Schüler is meant, I don't think that was the cause. At that time there were some cartridges with rebated rims in use. The cartridge 12,7x70 Schüler was also not a success at the time.

The matter takes place between the two world wars, so a lot is unclear. Nobody knows whether this cartridge was ever manufactured, let alone whether a weapon was built for it. The cartridge appears mainly on the well-known old catalog page.