The 28th Cavalry

Each company of United States cavalry had (and still has) a fork-tailed flag called a guidon. The US cavalry guidons had three designs on them; the first from 1834 to 1862, the second from 1862 to 1885, and the third from 1885 until now.

https://flagspot.net/flags/us%5Ecav.html

All three historic guidon designs, and many imaginary ones, have been seen in various cavalry and Indians movies, usually the wrong design for the fictional date of the movies.

Apache Rifles (1964) is no exception, having a fictional date of 1879 and using the 1885 and later pattern guidon. The guidon design adopted in 1885 is divided horizontally, red above and white below, with the regimental number in white above and the company letter in red below.

The guidon of Captain Stanton's unit says "28" above and "C" below, thus making it Company C of the 28th US Cavalry.

During the Indian wars period from 1866 to 1898 there were only ten regiments of cavalry in the regular army of the United States.

In real history the 28th United States cavalry only existed from 1943 to 1944.

https://en.wikipedia.org/wiki/28th_Cavalry_Regiment