I was unable to locate that marking the current edition of the "Broad Arrow." It has plenty of SA ownership stamps, and a quick internet seacrh turned up a page on South African Air Force history.
Here's the link:
https://www.defenceweb.co.za/wp-cont...-squadrons.pdf
And from page 16:
"In August 1946 the Air Gunnery and Navigation School was redesignated Bombing,
Gunnery & Air Navigational School (B.G. & A.N.S.) and moved to AFS
Langebaanweg to train pilots in navigational skills. The unit was redesignated Air
Operational Training School in early 1950 after it was decided that the previous
name did not represent the function of the unit adequately."
I think this above info, along with the South African ownership marking on your rifle, nails down that stock-disc marking. As to rifle ownership, it also has Indian ownership and prood stamps, so it's a guess as to which nation last owned it until it was suprlussed.