This „Springer“ double rifle is younger than you thought. It was bp proofed by the Austrian proofhouse number 3 in Weipert, Bohemia, now Vejperti, Czechia, in 1902 as the 2119th gun proofed that year. As Weipert was a gunmaking center just like Ferlach, the rifle was probably made there for Springer, Vienna.
While German proofhouses marked the narrower bore/land diameter of the barrel, the Austrians marked the groove or bullet one. The rifle is obviously chambered for the 11.6x65R D Express, DWM case number 191. As your chamber cast shows, the case is of the “D” shape with a quick taper at the base and more straight after about .5” from he rim. This may be a problem when trying to make cases from available .450 base, but “E” shaped brass. You will have to swage down the case body with a tapered ring die before fireforming.
According to the 1904 DWM catalog, the cartridge was loaded with greased .456 -.459 lead bullets, 300 -390 gr.