Gentlemen, what a fascinating discussion. Thanks for the skan reference. I love having better descriptive words, I would have used filigree before. Circassian? I'm not sure? Georgian in a Circassian style, though with the skan I don't know if I would even say that. The motifs that aren't skan seem too non-abstractly floral and make me think eastern Caucasus .There is also a chape present, though without a drag, which older Circassian sheaths did not normally have. On the original sheath galloon would have been a Circassian tell. JT, have you been able to inspect the wood in the sheath in any way to see if it is original? On the subject of a block ricasso, could the blade be shortened? How long is it? All in all I would say Russian or made for a Russian considering the essay marks or lack there of. I have read something concerning the neillo motif on the throat stating that this was favored by Russians, but I can't find the reference. Jim has an argument for pre 1896, but my gut says early 20th century Dagestani fittings with an older blade. I would love for some of our Russian friends to weigh in on this subject.
Now that I am done throwing out unfounded assertions. Three things popped into my head while reading this thread.
1) Jim in #6 the "watercolor" diagrams show the mountings that carry the blade edge up, but the 1877 does not carry vertically, over all they seem in a 19th century Caucasian style shaska configuration. Was this standard for Russian cavalry at the time, or just specialized for units in the Caucasus region, or simply artistic license? While a vertical carry was said to be good for moving on foot through brush and had obvious edge retention advantages it would seem to a bit of an awkward draw for a guarded saber.
2) Not all silvers tarnish equally. Depending on what the silver is alloyed with, often copper, effects how much the silver tarnishes and how quickly. Apparently humidity and exposure to sulfur have a large role in the chemical reaction as well.
3) I found this sight useful while having internal debates on which ivory I am looking at.
https://www.fws.gov/lab/ivory_natural.php