Geno:
The correct answer would be found at the Vienna Proofhouse and I'm sure the info is there. But I'd venture a guess the cartridge was based on the 450 3 1/4" Express and my 1st guess for the chamber would be the 450 2 3/8" or 11.6X60R which was a very popular Bohemian cartridge. That is if the rifling would span an additional 0.3mm. If only 0.2mm, then it may have been something like the 11.5X60R Mauser. And of course there are many other possibilities.
Can you see what the encircled initials on the watertable might be: "J.S.E."???
Kind Regards,
Raimey
rse