Chris,
Really, you could snap a picture of the barrel flats, plus barrels in front of the flats and behind the forend hanger, and have it posted here. This would clear up ANY doubt about what you have, and the wisdom (or possible risk) of continued firing full Nitro loads.
I think any potential buyer would expect such a photo anyway.....

Back before the 1904 Proof Laws there was no “Black Powder” proof mark, simply “View Proof” and “Definitive Proof” (London View Proof in your case maybe a London Definitive as well, you have not said as such) and for Express Rifles there was the mark as you earlier noted as “.450 EX”.
“If” the rifle was proved for Nitro at any time prior to 1904 that would only be noted by the grains weight of charge and “Cordite” as well as the caliber designation and case length.
The later 1904 Birmingham Nitro Proof mark you mentioned “should” be accompanied by the max grains weight of Cordite as well as projectile weight.
As to barrel weigh and profile.....that would have only little to do with BP versus Nitro simply because for a given desired length and weight of rifle the barrels would be of a certail profile no matter what. British sporting rifles of the 1890 era were sometimes done to what we would expect to be a Nitro Express weight even tho only intended for BP proof.

Given your interest in selling this rifle I would seriously suggest that you contact H&H to get any details they have on this rifle. They certainly would know if they had built a second set of barrels for it as well as the original proof history.
Any copies of records or provenance from H&H would certainly add to the saleability of the rifle.
-Mike