I have no idea just how old Bell is but I would suspect BP pressures were quite well documented long prior to his birth. I know they had been long prior to mine & I'm 73.
One factor of BP is its ability to burn at essentially a constant rate irregardless of confinement & pressure. Smokeless powder does not have this characteristic, which is why it has less restrictions on handling etc.
7625 was designed as a smokeless propellant to fit a particular niche in burn rates. The slowness of its burn in comparsion to some other shotshell propellants which makes it have a similar pressure curve to BP also makes it somewhat temperture sensitive, a factor not found in BP. It was not designed as a subsitute for BP.
Due to the temperture factor of smokeless as I want to be able to use any of my loads in cold weather if desired, I personally do not load smokeless to pressures below about 7500psi.
Others do so & seemingly get way with it, but I had some 7625 loads totally let me down in a duck swamp with temps in the low 20s F & the pressure of those were about 7K. Load was taken directly from old DuPont manual, current at the time of occurance.
