This is the Accuracy International 5 round .308 magazine for the AI AE Mk1 rifle (pre 2009).
A compact 5 round single stack magazine manufactured from heat treated steel with a Teflon coating, strippable for cleaning.
Please note that post 2009 AE Mk2 and Mk3 rifles use the AICS format, available in 5 round and 10 round sizes.