400,
A very informative post for sure.
If true it suggests *someone* was extremely reckless in their abuse of proof laws at that time.
What you're suggesting is that they were happy to offer guns proofed for a 16 ton cartridge yet chambered for a cartridge with the same ballistics yet smaller in capacity than the 17.5 ton 350 rigby magnum and therefore producing pressures WELL ABOVE 17.5 tons.
This would also suggest that anyone thesedays firing the claimed 2660 fps 350 No2 loads in double rifles proofed for the lower pressure 400-350, could also be being somewhat reckless regarding keeping pressures at safe levels.

Personally, I think there could be more to the story.