Definitely not a dip. Here are some better pictures with the gun assembled. Note the hammer and lever, which looks a bit more traditional. Different steel? I don't know. Also a before pictureI'm no expert but that looks like a chemical dip. I hope not if you wanted real bone and charcoal.
Respectfully, there are lots of ways to know. Old color photographs (Kodachrome came out in the 1930s) or movies. Old timers that remembered what they looked like when new (I don't recall ever reading anyone who wrote 'the colors in 1930 just don't look like those in 1880...') Then there are those that have fairly vibrant case hardening, from the 1860s to 1900, that were in cases, kept in boxes in storage, etc. If it's simply time or oxidation that mutes colors, then ALL of them from the same year would look the same. But some look good, some muted. By deductive reasoning, originally they had more of the look of the survivors. By the same token, the results of bone/charcoal case hardening are not THAT different today, or in 1975, or in 1965. And well preserved ones from 1959 look about the same today, as they did when new, according to owners. Why would a 1880s one that is stored in similar conditions be the only one to substantially fade?