SGT MABIN uniform facing color for "general staff"

What would his facing be

I am not sure if the "general staff" is created by picking officers and soldiers from other regiments currently available and creating a new organization, or if this is something permanant and that in theory the same general staff would provide staff services(?) to all the army.

if they are created by putting together a staff from the pool of regiments present, then Mabin would wear the uniform of that regiment. if so what regiment.. was he in the rifles

if it is a permanent "unit" in the army, then do they have a seperate uniform like say the commissary? if so what would the facing color be

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