And the plot thickens further.
My box of Gamebore Traditional Game 16g 28gm (1 oz.) which Gamebore tells me runs 1260 fps at 8122 psi (well below the 8960 service pressure) has written:
"These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4" or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
67 mm case length: Guns with a chamber length of 2 1/2" or longer, nitro proofed to a service pressure of 3 tons per square inch (850 kg per square cm.)"

Looks like there is a different modern 'standard' for 2 1/2" and 2 3/4", but as discussed in a previous thread, 2 1/2" chambers are indeed proofed at 850 bar.

https://sites.google.com/a/damascusknowledge.com/www/home