CJF,
It seems that you are mixing terms. In the 360-2 1/4" case there was an "express" loading with a lighter bullet at a higher velocity using black power. The term "nitro for black" doesn't apply to the rifles, rather it applies to ammunition loaded by factories for black powder rifles using smokeless power at a pressure appropriate for the rifles and hopefully to "regulate" close to the black power load. Also, in the 360 2 1/4" case, there was a Nitro Express load with a 300-grain jacketed bullet at a higher velocity for use only in rifles proofed for Nitro Express ammo. Your fine Watson double rifle is not for 360 Nitro Express, but 360 Black powder Express and 360 Nitro for Black powder Express should be OK.
Mike
