I also would say it is a magazine for the L85 Weapon series, judging by its black finish and RG (Radway Green) plastic floor plate. They can be modified by filing the magazine catch hole, modifying the feed lips and using a shortened (weaker) spring for use in the Marushin M16 series.
I have one myself I got from a friend in the UK (Claymore, was that you?) approx. 15 years ago ...