Guidons are a type of flag in the US army. Almost every company in the army has a guidon. A typical guidon is fork tailed, the color of the army branch, with the branch insignia in the center and the regimental number above and the company letter below, with the battalion number, if any, at the edge of guidon lance.
Except in the US cavalry, the only branch that used guidons back in the 19th century. From 1833 to 1862 the guidons were divided horizontally, red above and white below, with white letters "U.S." above and the company letter below. From 1862 to 1885 guidons had the stars and stripes pattern similar to the US flag. And from 1885 to the present cavalry guidons are divided horizontally, red above and white below, with the regimental number in white above and the company letter in red below.
In western movies the most common type of cavalry guidon is the 1833-1862 pattern, since the same prop could be used for different cavalry regiments, and the second most common is probably the 1885 to present pattern, which many movie makers could have seen in real life and newsreels. The design with the stars and stripes actually used in 1862 to 1885 when most cavalry movies are set was used much less often than the others.
In the scene where Hickok and Cody capture a Cheyenne played by Anthony Quinn who tells them about Custer's Last Stand, he has a captured guidon with the number "7" above and the letter "E" below, the guidon of troop or company E of the Seventh US cavalry. Company E was part of the battalion wiped out in Custer's Last Stand.
Earlier the Cheyenne ambush a troop of cavalry numbering 46 men under Captain Wood sent by Custer with ammunition from Fort Hays to Fort Piney. This troop is besieged for over a week and almost all the soldiers are killed or seriously wounded. Thus almost all of the men in the troop would be unavailable for duty when Custer rides out to fight Sitting Bull soon after.
And in scenes of the besieged soldiers I could see the letter "B" in the bottom half of their guidon. I couldn't see the Regimental number in the top half. If the company B that suffered severe losses was company B of the Seventh Cavalry that would significantly weaken Custer right before the Little Bighorn. In real life Company B of the 7th Cavalry was assigned to guard the pack train and participated in the Reno-Benteen fight on Reno Hill on June 25-26.
Note that in Little Bighorn (1951) a troop of cavalry racing to the Little Bighorn to warn Custer he is badly out numbered (and thus obviously not part of his column) has a guidon with the letter "C" and no regimental number. Thus they are troop C of some cavalry regiment. If they are troop C of the Seventh Cavalry (which was in Custer's Last Stand in real history) Custer will have one less troop than in real history.
In Warpath (1951) a troop of the Seventh Cavalry, probably Company M, suffers heavy casualties in a fight on a river island soon before the Little Bighorn. This fight, like the one in The Plainsman (1936) is obviously inspired by the Beecher Island Fight on September 17 to 19, 1868.
In Bugles in the Afternoon (1952) a troop of the Seventh Cavalry suffers heavy casualties - in a fight obviously inspired by the Wagon Box Fight on August 2, 1867 - soon before the Little Bighorn.
So in any fictional universe that includes The Plainsman (1936), Little Bighorn (1951), Warpath (1951), and Bugles in the Afternoon (1952) three of the twelve companies of the Seventh Cavalry that were in the Little Bighorn campaign in real life would have suffered heavy casualties soon before the campaign began, while a fourth may have been detached for other duties and not with Custer's command.
So in any wild west movie universe including those four movies the Seventh Cavalry would have been much weaker at the Little Bighorn than it was in real life, and thus the four movies can be thought of as unintentionally providing a fictional explanation for Custer's defeat that isn't valid in real life.