I have been reading a bit re the older blackpowder cartridges, especially the bigger ones such as the 45/70 and up. Ran across a reference to a group of loads using 5-6 Grs of a powder referred to as "PB" and then a case full of black powder compressed to allow room to seat the bullet.
What was the exact purpose of the smokeless powder under the black, better ignition? That is the only reason I was able to surmise.

I dont know for sure,but I think they used to load some smokeless to raise the pressure,at least,I have heard of it.
I dont think any smokeless powder ignites as well as black powder.

Duplex loads in black powder applications are much cleaner burning. I use 2400.

Not for better ignition; Artillery powder charges use black powder to get the smokeless started. It is to clean out the fouling. Lyman did a lot of testing and up to 10% of the BP charge, in smokeless, is ok, but above that, pressures start to rise fast. They used 4759; I usually use AA5744; a friend uses 2400. Not more than 6 grains for a 60 grain BP load. Bear in mind that the NRA does not allow duplex loads in silhouette competition, nor does SASS.

Just for the record:

BP firearms ALWAYS are marked, "Black Powder Only."

I've never seen any sort of firearm propellant which wasn't marked, "Do Not Mix Powder Types."

But then I've seen people stuff the muzzle with a smoke hanging between their lips.

"Clean-up on the shallow end of the gene-pool!"

I use 4g of H4227 under 60g (by vol.) of compressed Kik 3fg black in my Marlin 45-70. Bullet is Lyman 457193 420g FN. This a "hot" load (about 1500fps) so don't try it in anything but a very strong rifle. I can shoot 20 of these without cleaning with no loss of accuracy. Without the H4227 accuracy is gone after 4 or 5 rounds...even using a good black powder bullet lube. Duplex loads have their place but I only use them in the Marlin for hunting.