Not strange at all, seeing as the “dummies” were made with cases surplus or not up to quality standards for Loading as Ball. So a US type AN-M2 case was made into a British Contract Dummy (Green Filled Flutes).
BTW, the Green was probably to distinguish the .30 cal case from the BESA 7,92mm Drill, which was Red-filled flutes; the two cases/cartridges are commonly confused, even with Ball, with disastrous results; even though the .30 was almost only used by RAF until the formation of the Home Guard in mid- 1940, and the HG supplied with US .30 calibre weapons (M1917, etc), and the 7,92mm Calibre was restricted to the Tank Corps/Armoured Corps.
The LoC (List of Changes) should have some indications as to the design of the Drill (“Dummy” )Round for the RAF.
An interesting area of British Ammo use of the Early Wartime years.