If Fort Apache, She Wore a yellow Ribbon, and Rio Grande are a trillogy, the same cavalry regiment might appear in all three. So close observatin of the insignia on Captain Brittles's kepi and perhaps others in she waor e a Yellow Ribbon, and the kepis wore by the boys in the rescue scene, and perhaps other scenes, in Rio Grande, might identify the number fo the regiment or regiments involved.Captain
shareI have seen Rio Grande again recently and could see that Colonel Yorke, Sergeant Major Quincannon, the singer played by Ken Curtis, and others wear caps with crossed sabers and the regimental number 2. Thus the regiment should be the Second United States Cavalry, as in She Wore a Yellow Ribbon where several characters also wear caps with "2".
But in the scene in the abandoned church where 2 boys look over the top of a pew one wears a cap with a "2" and the other one wears a cap with a "7". I have seen a still where Patrick Wayne, John's son, wears a cap with a "7", so I guess he's the boy. therefore one of the kids in a fort where 2nd cavalrymen are stationed should have acquired a 7th cavalry cap somehow.