BPE does indeed mean Black Powder Express.
There are nitro for black loads available to the reloader. Be aware the dies are a bit more expensive than more readily available calibers.
In a single barrel rifle where you don't have to worry about regulation you will be able to successfully use blackpowder or nitro. The cases you will be purchasing are actually .450NitroExpress (.450NE) cases, so you should also be able to step up the velocity, if you so choose, over BPE norms.
FWIW that stock seem to be oddly shaped but it is hard to tell since there is no picture of the entire rifle.
