The 6,5 MS Cartridges marked “FN 50” were made on order of the Royal Hellenic Army during the reconstruction phase of Greece’s Political and military Structures after the ruinous Civil War (1944-48) between The Communist Partizan Groups and the Royal Hellenic Army and Monarchist Partizan Groups; The Greek Communist Party had hoped Stalin’s Red Army would assist them in taking over Greece; but the Yalta accords between Britain, USA and the USSR, placed Greece squarely in the “British Zone of Influence”, so Stalin “stood Pat” and allowed the Greek communists to be defeated.
BY the 1950s, Greece had received thousands of British Rifles, MGs etc and Millions of rounds of WW II .303 ammo; the PCH factory at Piraeus was in a sorry state after the depredations of both the Germans and the Civil war, so ammunition for the older MS rifles was needed to be “bought in” to cover the change over to .303 and Later, .30 cal US.
The Contract was fairly Large, as much of the 6,5 ammo was later surplussed off (through Interarms) in the 60s and 70s ( despite GCA68) as were the entire MS rifle and carbine inventory after 1986.
The 6,5 ammo was snapped up by Hunters with both Sporting MS rifles and by Military Collectors, as it was the only source of shootable 6,5 MS ammo then available (Norma and RWS were much too expensive.) It had mostly disappeared by the Late 90s, with only the occasional Packet showing up as “collector’s Ammo” at Gunshows. BTW, the stripper clips for 6,5MS ammo are Unique in size.
Some US “ammo tinkerers” converted these rounds to be used in 6,5 Arisaka, by resizing the primed case, trimming it, then reloading the powder and bullet. As mentioned above, the converted case was made to HS on the shoulder; —the Arisaka Cartridge does actually Headspace on the semi-rim; Have done this in a Take-off barrel when “popping” the Primers to make deacts for Japanese buyers ( I used a rounded Nail-set Punch the diameter of the Primer (.200") in a sleeve over the barrel threads, with Face mask and leather apron and gloves–barrel held vertically in a vice (Muzzle down).