An Infantry flag?

In several scenes in Arizona Raiders set in 1865 and 1866, cavalry carry a highly unusual guidon - a fork-tailed flag carried by each company in the US army.

In western movies the most common type of cavalry guidon is the 1834 to 1862 pattern, red above and white below, with white letters "U.S." above, and the red company letter below. The second most popular type of cavalry guidon in westerns is the pattern used from 1885 to the present, red above and white below, with the regimental number in white above and the company letter in red below. A very distant third most common type of guidon in western movies is the type actually used from 1862 to 1885, with the stars and stripes on the fork-tailed flag.

But the guidon seen in Arizona Raiders is not any type ever carried by the US cavalry. It is all black or dark blue with hard to see designs in a lighter color. In one scene the number "18" is glimpsed on the guidon, implying the soldiers are in the 18th US Cavalry.

Seeing it again on 11-11-2017 the guidon looked like it had the number "18" above crossed sabers (for cavalry) or muskets (for infantry) which in turn was above the number "1" or the capital letter "I". Thus it would supposedly be the guidon of Company I of the 18th Cavalry.

The modern US infantry uses dark blue guidons with the crossed rifles of the infantry branch and the regimental number above and the company letter below, all in white.

Thus it seems likely that the person in charge of props for Arizona Raiders (1965) - probably property master Charles Henley or art director Paul Sylos - either acquired a guidon of Company I of the 18th US Infantry or else had served in the infantry and thought that cavalry guidons looked like infantry guidons.