Just looked at the GI advert photos again, and expanded the frame for a closeup of the left side action bar......

Very clearly engraved: “CHARGE 4 1/2 DRAMS, 500/450, 3 1/4” CASE”.....that is a BP load.

The rifle in question is unarguably ORIGINALLY a .500/.450 3 1/4” Black Powder Express, while made to a heavier weight appropriate to the “Magnum” BPE.

THIS IS IMPORTANT:
Yes......BPE rifles were reproved for Nitro, no doubt about that, and fairly common. Quite likely for NfB or Light Cordite though. Be aware that full Nitro loads are fully 50% higher pressure than the original BP loads or the equivalent NfB/Light Cordite.
The issue? Full Nitro equivalent to the 75 Cordite 480 grain load could beat the rifle loose in short order.

Any Nitro Proof marks should show reproof “crown/R” along with the seller stated “crown/NP” from Birmingham on or after 1904.
Very curious now as to the load for which the rifle was reproved? NfB? Light Cordite? Full 70-75/480 Nitro?
Once again, the barrel flats WILL tell the tale if properly reproved in England.

Best of luck on your sale.....as stated, a nice looking rifle.
- Mike