Mike,
I have my reservations about Moore's "Sergeants Musket" attribution. His book was published in 1967. There was almost nothing available at the time on the subject of French arms except Hicks and that goes back to the 30s. The French texts I have, especially Boudriot's "Armes a Feu Francais" don't mention a sergeants musket at all although it attempts to list all of the regulation types. It does, however, include information on officer's muskets which, unlike as in British service, were Ordnance manufactured & issued as regulation arms. Actually, Moore calls it a "sergeant's or officer's musket".
Unfortunately, I don't read French very well so I don't know if the brass plating is mentioned. It does seem as if this might be something else as this is too early a date for electro-plating and close plating, as in Sheffield plating, would be difficult, very expensive was usually done in silver and, as far as I know, not done in France. Much more so than making brass barrel bands. I have noticed that many so-called "gilt" swords of the period are actually heavily laquered so I wonder if this is what both you and Moore are seeing. They could also have been fire gilt and laquered which could have the appearance of brass.
Joe Puleo