Hello pelle,
A very nice rifle you have there.

In regards to shooting, the .450 3 1/4' Express brass cases would be all the same, makes no difference BPE or NE.
That said there can be rather significant differences in the actual chamber because there were no official chamber dimensions across all makers back in those days. all you can do is source some proper .450 brass (thin rim) and see if they will chamber.
If not, the solution is custom sizing die from a chamber cast, possible alteration of the actual case, or having the chamber reamed with a current CIP standard chamber reamer.

Also keep in mind; the actual bore/groove diameter may not be the current standard ~ .458" in the grooves. Some tighter, some larger, in the black powder era.

Enjoy the journey to get your H&H shooting.
- Mike