Looks like the double shoulder the FG42 leaves after firing.
I came across this thread when looking for something else, and can add some detail. Cartridges externally visually similar to Jim’s OP and packed in the boxes described above by Fede were made by Kynoch and supplied by the British government between 1964-1965 for use in Aden, either direct to Crown Agents in Aden, or via British Ordnance Depots in the UK. 4,000,000 ball rounds were reported to have been produced in this contract. The reference described the bullets as flat based 154 grain with GMCS envelopes, which differs from Jim’s specimen which appears to have a standard 2z bullet (Ref: ‘The Development in Britain of .30 inch Ammunition, 7.92mm Ammunition, 15mm Besa Ammunition 1939-1993’, Peter Labbett, 1999.)
I’ve taken a couple of images of the ‘Aden Contract’ box as described above for the record. Beige card box, card dividers internally, pale yellow label with black print, and clear sticky tape on the front to close the flaps, which runs behind the label. All the best, Pete.