Dear WBD,
Yes, the brass (but not the bullet) of 10.75x65R Grundig is the most likely. It is possible to put it into the chamber (see photo). This brass would be the best choice to use it for reloading and shooting, but after fireforming.

The diameter of the bullet in Grundig cartridge is too small for my barrel (10.695 mm 0.421). Also the precise 10.75 (0.423) diameter will be too small. I need the bullet with diameter a little less than 11.05 (0.435). May be 0.434 or 0.433 – close to 11 mm.