We need to keep a couple of things in mind when we think we know what we know, or when we're putting together uniforms for an impression. The original post asked about 1877-1878. That's pretty wide time range. A soldier could easily have one set of equipment in April of 1877, and something completely different in April 1878, with endless variations before and after. If you want to be totally correct, you'd almost have to pick one individual on one given day, with a photograph to prove it. What most of us really do is pick sort of an "average" individual on an "average" day during the time period and place selected.
Another thing that occurred to me is this: I think we all know that the most reliable source material we have is period photographs. You can't argue with what is actually in the photo at a specific time or place. We need to remember, though, that it's still a very limited sample. If you have a photograph showing five men in a hundred man unit, you only have a photographic record of five percent of the people there. That means you're not sure about what the other ninety-five percent of them have. We can make very educated guesses on what was where, but I'm not sure we can ever make totally definitive statements.
One other thing. Even if an entire unit gets new equipment, there's always going to be that one guy who still uses his old one, just because he likes it better, or maybe just as his own little protest against conformity.
Just my thoughts, as always.