DANCES WITH WOLVES takes place during the 1861-65 War, and SWAYR takes place in 1876, yet in both movies we see cavalry greatcoats with a yellow lining, an outfit introduced in 1883, during Arthur's Presidency.
God is subtle, but He is not malicious. (Albert Einstein)
And there are many more such anachronisms in cavalry movies, westerns in general, and historical era movies in even more general.
For example, the guidons or fork-tailed company flags carried by the cavalry. The US cavalry carried 3 different designs of guidons, 1833-1862, 1862-1885, & 1885 to the present. And there is almost literally not a single example of a cavalry movie using the correct pattern of guidon for its fictional date (if any fictional date is given). Some cavalry movies use guidon patterns which never existed in real history.