Unusual .303 Blank

I have just acquired this .303 blank by Royal Laboratory. The picture is not very clear, but the headstamp is R^L at 12 o’clock and a faint “II” at 3 o’clock. The mark at 6 o’clock is only a case blemish.

The round looks as though it may have been copper washed, but this could also have been caused by zinc leaching if the round has been in the ground (although it shows no sign of this).

Any ideas?