Andrew:

That's a lovely old Manton.

Your rifle is a .500 BPE. It was not specifically proven for Nitro-for-black, as your rifle was built before that loading was introduced and before Nitro Proof was available. Your rifle was proved in London between 1875 and 1887. The bore was marked in gauge - "39" - at the proof house rather than in English decimal - 500. Notice that the "500" is marked only on the right barrel. That isn't a proof house mark. Proof house marks have always been made on all barrels, not just one. The marks of the London Proof House are those duplicated on both barrels and the action flat - there should be a "view" mark (crown over V) on both sides of the water table.

Britain's proof houses marked the bore size of rifles and shotguns BOTH in gauge until the new rules of 1887, at which time the mark for rifles was changed to the nominal bore in decimals, thus "39" became "500" in 1887.

Nitro Proof was introduced under the 1887 rules. The famous nitro proof MARK wasn't introduced until 1904. A rifle specifically proved for the Nitro-for-black loading under the 1887 rules would be stamped on the flats of both barrels "Cordite 55grs - 440grs bullet max'm" and there would be no Nitro Proof mark. After 1904, the marks would be "Cordite 55 - 440MAX", with the new Nitro Proof mark added. The marks on your gun are precisely those of an ordinary BPE proved before 1887.

Further, the Nitro-for-black loading gets a bit confusing.

That isn't really correct. A rifle specifically proved for Nitro-for-black is no different from any other rifle proved only for the black powder version of the cartridge, as there was never any difference in the proof standards of the two. Nitro-for-black means exactly what it says - nitro powder cartridges specifically developed for, and intended for use in, rifles built and proved for the BLACK POWDER CARTRIDGE. By British proof law, these cartridges could not develop greater pressure than the black powder version, or they couldn't be marketed as "Nitro-for-black". Technically, any BPE rifle that's "in proof" is a Nitro-for-black rifle, as that is what NFB cartridges were developed for. The fact that a few BPE rifles were specifically marked as proved for the NFB load back in the day resulted in considerable confusion on that point over the years.

If the bores are as good as the rest, you have a keeper.