I read it as 7.8 / 58. IMHO it's more likely a 8x58R "Sauer & Sohn", a tapered case with a base diameter 11.05 mm = 435". Knowing the service powder charge, stamped just under "St.M.G." as "?.? g G BlP", would help identifying the cartridge. Sorry, but I can not read it in your photos. The drilling was proofed in Suhl between 1912 and 1923. Will try to identify possible makers.