The 2nd Cavalry
Winchester '73 opens on July 4, 1876 at Doge City, Kansas, and ends a few days later about 200 miles away in Tascosa, Kansas.
In 1876 four companies of the 2nd US Cavalry were stationed in western Montana and were part of Gibbon's column in the Great Sioux War. The other eight companies were stationed in Wyoming and probably all of them were part of various expeditions during the Great Sioux War.
There were 5 companies of the 2nd cavalry from Wyoming at the Battle of the Rosebud 17 June 1876, for example.
In the Cheyenne episode "Gold, Glory, and Custer: Requiem" (11 January, 1960) Crazy Horse or Dull Knife shows Cheyenne a cavalry guidon (flag) with the number 2 on it while boasting about the Battle of the Rosebud, correctly implying that part of the 2nd cavalry was at the Rosebud.
In the "Massacre" episode of The Time Tunnel (20 October 1966) Company B of the 3rd Cavalry is mentioned as being part of General Crook's command, and in fact it was at the Battle of the Rosebud.
But Chief Crazy Horse (1955) inaccurately shows a cavalry guidon marked "20" and "A" at the Battle of the Rosebud.
As far as I know, no part of the 2nd US Cavalry was stationed on the southern plains or in the Southwest during the 1870s. And the soldiers of the 2nd Cavalry could have been thankful that at least they didn't suffer from the hellish heat of the Southwest. It is said that General Sherman considered the Southwestern climate so bad that he once suggested that the USA should fight another war with Mexico to force Mexico to take back Arizona and New Mexico!
But in the the movies it is different.
In Apache Drums (1951), in the year 1880, Victorio's Mescalero Apaches are believed to have crossed the border into Mexico, but when Lieutenant Glidden arrives at Spanish Boot, New Mexico, he says that Colonel Evans of the 2nd Cavalry cut their trail back across the border last night. The close captioning says 2nd Cavalry, though it has been wrong; when Apaches speak what sounds like Spanish to me the close captions say they are speaking a native language.
In Rio Grande (1950) several soldiers at fictional Fort Starke in the Southwest near the Rio Grande wear army caps with the number 2 above crossed sabers, signifying the 2nd Cavalry. The fictional date seems to be 1879 or 1880.
In She Wore a Yellow Ribbon (1949) Fort Starke seems to be somewhere on the southern plains as Cheyenne warriors come south after Custer's Last stand on 25 June 1876 and try to incite the local tribes to rise up. And several characters wears caps with 2nd cavalry insignia.
A cavalry unit, Company D, from fictional Fort Starke is in The Command (1954) with a fictional date of 1876 or possibly 1878. For consistency with She Wore a Yellow Ribbon (1949) it should be a company of the 2nd Cavalry, but I couldn't hear if they said 2nd Cavalry or 7th Cavalry.
And in Winchester '73 there is a fight between cavalry and hostile Indians under a Young Bull somewhere between Dodge City, Kansas and Tascosa, Texas, 200 miles away, a few days after July 4 and when white and red characters are first hearing about Custer's Last Stand.
I could see the company letter "A" on the guidon of the cavalry but not the regimental number. But a wagon cover has lettering saying something Like "US Army Quartermaster" and then "Company A, 2nd US Cavalry". So apparently in Winchester '73 "Company A, 2nd US Cavalry" was stationed at nearby Fort Bascom, New Mexico in 1876.
And in real life Company A of the Second US Cavalry was hundreds of miles north of there, being at the Battle of the Rosebud on 17 June 1876.