The yellow is simply age related. MOP yellows over time and yours are over 120 years old. All 32 HE, 1st Model revolvers are classified as antique by the BATF, since frames were all produced before 1898. Your revolver would have shipped around 1898, so the stocks may or may not be original. I see a chip on the right side stock that can be easily filled with epoxy. If you could find some MOP (the inside of oyster shells would work) and drilled a few shallow holes in it, you can repair that area quite easily.
Use the drill shavings and mix it into the epoxy, adding enough to make the epoxy opaque. Use tape to create a dam and carefully fill the crack, building it up just over the surface of the stock. Using a small fine needle file, level the area after full cure and use 400 grit sandpaper to sand smooth. Using toothpaste of auto paint polishing compound, you can buff the area and will substantially improve the appearance. Make sure you drill the clam shell outside, since the powder can be dangerous to inhale.
