Johann Sigott, Kirchgasse, Ferlach house number 34, lived from 1872 to 1955.
A chamber cast will certainly answer the question about caliber, but a 12.7x60R aka .500 2 3/8” cartridge was a quite popular sporting number in pre-WW1 Austro-Hungary. A lead bullet, blackpowder cartridge with no British equivalent, it was made until WW2. 12.5 mm may stand for the bore, not groove or bullet, diameter. Or it may have been made slightly undersize for best accuracy with the factory greased lead bullets.