the "USE IN SA ONLY" indicates you have a 2nd class receiver. Specifically, the receiver has a machining error, is out of tolerance and, therefore, fails to meet interchangeability requirements, and couldn't be shipped out to another arsenal or depot as a spare replacement part. The receiver would require some custom fitting or parts selection to fit properly. Nothing went to waste. So rather than scrap the expensive receiver, SA would manufacture or hand-fit a custom part to the receiver to overcome the machining error.
I'm impressed by the fact that they had the foresight to stamp C-R of the receiver...
C&R stands for "Cleaned and Repaired"
C-R was a joke.
That they knew ahead of time that the rifle would be C&R (Curio and Relic) one day.