That picture is in my copy of an Osprey book about the ISDT - it's captioned:
"Eric Chilton's 650 BSA being prepared for the 1958 event. Riding in the BSA works team Chilton's machine had a Road Rocket engine with reduced compression ratio in a Gold Star frame. He was destined to win a gold medal"
So in fact an early RGS!
The most notable BSA participation in the ISDT was perhaps the 1952 event where 3 stock Star Twins rode about 5,000 miles across Europe including winning 3 Gold Medals and collecting the Maudes Trophy.