Re: the GunsAmerica advert pictures.
The “25” with the original proof marks is the bore diameter in gauge (.571”) and was used until 1887 at which time barrel bore size stamping was changed to decimal. (ie: 577 EX)
So the barrel was proofed for black powder no later than 1887. The later roughly stamped “.577 EXPRESS” has no bearing on existing proof and this rifle is not Nitro Proof. Safe “Nitro for Black” loads will still work.
Buyer had better slug the bore before they shoot it, even though .585 bore is advertized, the actual groove diameter could be as tight as ~.579-.581 and could possibly have originally been a .577 Snider.

My personal thoughts: the asking price is astronomically silly.....1880’s rifle (or at least barrel), modern refinish on metal & wood, not a Nitro rifle.
- Mike