Ben:

$4,500 US would be a very low figure for an original .500 Nitro hammergun to be sure. It would be low for a .500 BPE for that matter. Check it out carefully. An original full nitro .500 would be marked "Cordite 80, 570 MAX" on the barrel flats.

Be careful of the Nitro for Blacks. A few rifles were specifically proved for the NFB load, and are therefore "Nitro Proof", but not really. The .500 NFB load ranged from 52 grains Cordite with a 340 grain lead bullet to 63 grains Cordite with a 440 grain lead bullet. These rifles will have a Nitro Proof mark if made after 1904. If made prior to 1904, Cordite will be mentioned on the flats but there will be no Nitro Proof mark. Either means Nitro proof. However, Nitro for Black cartridges were specifically intended for use in Black Powder proof rifles and did not develop greater pressure than standard black loads. In other words, Nitro Proof for Nitro for Black loads really means Black Powder Proof only. Don't confuse this with true Nitro Proof. You can't go by the Nitro mark, you have to look for the load marked on the flats along with it.
---------------------------------------------------------------------------