Raimey,

Felix gave some numbers here:
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=459762#Post459762
In 1900, the Kalezky shop sent 19 guns to the Vienna proofhouse, 16 of these were shotguns.

That wax cast resembles a 11,5x65R or.450 Express, german form (rim diameter ca. 15.5 mm, head diam. 13.78 mm, quickly tapering, rim thickness ca. 1.06 mm, measurement taken off a GR / Roth case).
But this chamber apparently wants a much thicker rim.
Since you had a .45-70 cartridge at hand - would that chamber?

fuhrmann