No matter how a double rifle has been marked The meaning is still the same! A black powder double rifle is shootable with NITRO FOR BLACK loads! A "Nitro for black" load is loaded with a smokeless powder to the pressures developed by its full black powder loads and no higher.

I see no reason for confusion here! A Nitro for black load is a reduced load with smokeless powder, usually Cordite but they can be loaded appropriately with any smokeless powder that will produce the black powder load pressure. The rifle doesn't have to be proofed for a NITRO FOR BLACK load!
...........................................Why is that son hard to understand???????