This is the first of the Rossbach French infantry flags.
First raised in 1629, St Chamond (or St Chamont (as on the 1757 plate) or St Chamant) had 2 battalions and in 1757 was part of Soubise's Army of Saxony. The grenadier company distinguished itself in the chaotic fracas at Weissenfels 31st October 1757, helping to cover the retreat of the Bavarian and Palatinate troops from the town across the bridge and delaying the pursuing Prussians while the bridge was fired, and then escaping across it before the bridge took fire. (Christopher Duffy gives a good account of the action in his book Prussia's Glory, pp.56-59. It seems from his account that it may have been an officer or officers of this regiment stationed on an island in the river who spotted Frederick reconnoitring the burning bridge and who reported to their commander the Duc de Crillon that it would be easy to pick him off from there. de Crillon told them they were there simply to observe the destruction of the bridge and "not to kill a general who was on reconnaissance, let alone the person of a king, which must always be held sacred". How different the course of the war and European history would probably have been if he'd said "go ahead and do it"!) The regiment was at Rossbach, brigaded with the Cossé-Brissac regiment. St Chamont was in the left hand column of the three infantry columns just behind Piémont which was in the lead and suffered heavily there, losing 400 men and having its colonel wounded. For most of the rest of the war the regiment was in France, recuperating and after that manning the coast of Normandy and then Belle-Isle.
The flags as depicted were carried by this regiment from 1749 to 1762.
And this plate shows the uniform and flags in 1757: