Files-on-Parade is a single man. He was the first man in the first squad of the company and the rest of the company "guided" on him. He man even have been in front of the First of the first. It was he who set the line on which the other men "dressed their ranks".
He ahs also been called the "right guide" in other armies. I am not familiar with the disposition of troops on parade in the British Army during the last 19th century to know the position of the Colour Sergeant but he must have been close enough to "Files" to talk without being observed by the officers or -infinitely worse- the Sergeant Major.
I'll dig out my old drill manuals and see what I can find.

troll