South African contract SMLEs

I'm in the process of buying a SMLE made by BSA, it has the mag cut-off, button cocking piece and windage rear sight. It has no Crown or date stamps but has the South African "U" stamped on the knoxform, the serial number has no letter prefix which again points to SA issue. According to the Stratton book, because of the lack of letter prefix, this rifle was sent/issued in 1916.
Does anyone have information if this was common to SA rifles to 1916 only, or were they sold to SA post-WW1 minus the prefix letter, and how many SMLEs were sold to South Africa.

South Africa was not a separate country until 1931. Basically a Dominion like Canada had been. Any No. 1 Rifles would have just been sent there.
The South African Overseas Expeditionary Force in W.W. I was an all volunteer force comprised mostly of men who enrolled as individuals not as units. SA law wouldn't allow the Union Defence Forces to leave Africa.
These guys will know how many rifles were sent.
http://samilitaryhistory.org/