It looks very similiar to a kit holdall. And certainly similiar if not indentical to something i was issued in 2001 but in dark blue. We used them for holding personnel kit that would be used over a 48hr period (For example PT kit, wash and shave kit, spare combats and boots, etc).
No idea if it was used in the same way back then, but hope it is of use.
Interested in anything related to Auxiliary Wiltshire Patrols and the Wiltshire Regt.