Thanks for the information, its very interesting to know.
I notice that in the 1756-63 Campaign, as Prussia, the first Musketeer Btn you create is I/Forcade de Biaix IR23. Which is the first available name in the RoP_DB_Units file. Curiously this name is available in the DB because in the set-up files this Btn is named I/Forcade de Biaix IR6 in error (I think there is a separate thread on this).
Anyway, looking at the DB files, Prussia will create new units (for Fusiliers) starting with I Btn for IR50 & 51, then, II Btn for IR50 & 51, and then will create III Btn's starting at IR34 through to IR51. This is all assuming that no starting Fusilier Btn units have been destroyed.