Throughout the film, Colt Single Action Army revolvers (commonly known as Peacemakers) are used by various actors including John Wayne, Roy Rogers, and George 'Gabby' Hayes. This revolver was not produced until the 1870s. The film is set in the late 1850s and 1860s.
Observing Cantrell's uniform, Bob Seton tells him, "General Beauregard and the Fifth Army are a long ways from here, but that doohickus on your collar says Fifth Army." In the Confederate Army, collar insignia only indicated an officer's rank, not which general's army he belonged to. Furthermore, Civil War armies were not numbered like U.S. forces in the twentieth century, but instead took their names from states, regions, rivers, etc. (ex. the Army of Tennessee, the Army of the Potomac).