I’ve reviewed both my collection and files on South African 7.62x51mm ammunition from the 70’s and the M59 box is not consistent with other RSA boxes of this era nor is the M59 designation. Pre-1973, Pretoria Metal Pressings used the designation Ball MK 1 or Ball A1. From 1973 onwards, the manufacturer was designated by the letter A which in turn used Ball R1M1. Examples of the labels are shown below:
The only other example of RSA-associated ammunition using the M59 designation is a label shown in a old RSA Cartridge Collectors Association newsletter for linked ammunition using Ball M59 lot 5/76 (no headstamp shown).
With regard to the use of (+) in the headstamp, examples dated 74, 75 and 76 (is this lot 5/76?) exist. Shown below are 74 and 75 alongside the usual 74 headstamp (An FN-made cartridge is included for comparison).
As already stated the A (+) 7.62 cartridge is not an M59 despite the box label (it does not have a steel core). The casemouth is sealed with a black tar like material that I've seen on other RSA ammuntion from this era and the powder type and charge is consistent with other RSA ball ammunition from the 70's. Interestingly, unlike other RSA 7.62mm ammunition which use staked primers, the primer on this cartridge has a ring crimp (the A (+) 75 7.62 and A (+) 76 7.62 cartridges also have staked primers).
Careful comparison of the headstamps suggest that the A (+) 74 7.62 has more in common in style to the A 74 7.62 R1M1 than FN-made cartridges. However, I'm looking at just one box of 20 cartridges!
So back to the question, was the M59 box and its contents made in South Africa for internal use or possible export, or was it made by third party for South Africa? I don't have an definite answer, but tend to favor made in RSA for export.