Looks like these coins were struck in small numbers during the 1950s - how were they distributed? I assume they didn't circulate but were somehow sold to collectors.
The South African Mint had one pound and half-pound gold coins dated 1952 for sale in 1964. South African Mint pricelist 1964 The mint could not sell them to United States residents due to the US gold regulations.
Before the recent runup in gold prices, these were some of my favorite low-mintage world gold pieces because they didn't command the kind of price premium that the low mintages would imply.
Interesting - elsewhere I've seen that they were sold as part of "long" proof sets but it looks like loose coins were sold too.
