Cordite loads came along in this cartridge sometime after that date. 1889 was actually the year of development for Cordite and as far as I know Rigby with the .450 NE, approximately 5 years later was the very first sporting cartridge using it as a propellant. Can we see the flats? Do they they carry NP marks? If so then it was proofed for cordite after the fact as the NP proof came into effect in 1904. I have seen original Rigby Nitro guns built before the NP mark and they were very prominently marked(in Gothic scroll like only Rigby does)on the barrels by the maker that the gun was a special big game rifle for use with cordite. Do you have a letter from H&H? It is looking more and more like a 500/450 Express, not a Nitro express. I have been wrong plenty before but as I see it either this gun was built as a BP gun or your date of manufacture is incorrect.

Still a pretty gun though and who does not love a good H&H?