Yes, the 9.3x63G is based on the 12.97mm =.495" 10.75x68 case. Effectively it is the older 10.75x63 case necked down. The 9x63 Florstedt case was instead based on the standard 12mm = .473" Mauser base, which makes it so similar to the later .35 Whelen.
Apparently the same rifle was described by Mats Bergholm in the Dec.2005 issue of the SA MAGNUM magazine. The loads mentioned by Bergholm gave 2580 fps with the 286gr Norma Oryx or the 320gr Woodleigh at 2435 fps.