Never heard of a Franz Schubert, but there was a Fritz Schubert at Märzenberg 37, Zella-Mehlis in the 1920s. Please, post photos of the markings and proofmarks.
At .264" groove, your barrel is quite wide for a 6.5x58R Sauer & Sohn. German specifications called for a groove and max bullet diameter of 6.64 mm = .261", so DEVA loads are pressure tested using bullets sized down to .261". Max allowable pressure of the 6.5x58R is 2800 bar (piezo-electric transducer).
These loads are listed in the DEVA handbook, using powders available in the USA:
Sierra 1700 .264" 85 grs HP
IMR 4064: 29 grs for 2510 fps at 2408 bar
VV N140: 28.5 grs for 2560 fps at max pressure
Hornady 2610 .264" 100grs interlock
IMR 4064: 26.5 grs for 2330 fps at max pressure
VV N140: 26 grs for 2395 fps at max pressure
As both cartridges are very similar, you may also use .25-35 Winch. data as a guide IMHO.
