Gert
I think that rifles issued by the Oranje Vrystaat had an O.V.S. marking on the left side of the action wall, close from the receiver ring. The year of issuing (or of purchase?) was also stamped below the O.V.S. marking.
With regards to rifles issues by the Zuid Afrikaansche Republiek (Transvaal), I think that they had had an A, B, or C letter prefix preceding their serial number.
Many privately-owned sporting Mauser of different models, not bearing any kind of Government marking, were also used throughout the 2nd Boer War (1899-1902).
Best regards.
Louis