Zmaj
I am an engineer in the South African aerospace defence industry. Unfortunately, I should warn you that good info on SA weapons projects over the years are incredibly difficult to obtain in the popular media, unlike the weapons produced by most other countries. Since many of the older projects were specifically aimed only at local use, the security around these have always been extremely tight and there has been very little need for glossy-type advertisement of the products. Even at armaments shows you will usually only see a mockup of the weapon with a bit of general info such as "High Agility Short Range IR Guided Missile".
When you do research in the open literature, the best source I can recommend is material written by Helmoed-Romer Heitman, a defence correspondent who always had very good inside info in the SA industry. He will also usually distinguish clearly between official figures and estimated figures. However, note that even he has to rely only on unclassified info.
The majority of material that I have seen on the web and even in books on SA missiles in particular is extremely inaccurate. Unfortunately, due to the mentioned security even surrounding the now withdrawn from service missiles, I cannot give you much detail besides the few points below:
You asked about S-Darter and T-Darter specifically:
In the case of S-Darter, one should be careful. There was a missile called V3S (actually still in service), which is often referred to as the "Snake" in SA service. It is still flown on Cheetahs, although a lot of them get fired in air-air exercises these days as they are now reaching the end of their shelve life. These missiles are simply Israeli Python 3's - chosen since they could easily be integrated with the HMS used by the SAAF since the late 1970s while the all-aspect V3C was still under development to replace the rear-aspect only V3B.
The V3C never really reached true "mass production". Since not all V3Cs looked the same due to changes during the early production period, they are often referred to under different names in the media which is a cause for much of the confusion.
The missiles referred to in the media as S-Darter and T-Darter are Meteor class BVRAAM missiles with air breathing engines - speculated to be under development. I think the names S-Darter and T-Darter got published when Pakistan had some interest. You will find very little about these missiles anywhere in the media, and you should view anything you read about them with great suspicion - most of it will just be speculation.
The A-Darter is the new generation short range IR AAM currently in development with some joint participation from Brazil and a bit of interest from India - by far the majority of the work is done by the SA industry though. It is also the biggest and most expensive missile programme currently underway in the South African industry. This missile is more-or-less in the same class as Iris-T and it is specifically aimed at integration onto Gripen together with the new Gripen HMD. BTW, the Gripen HMD currently under late pre-production testing is probably the most impressive HMD available right now, but I digress... Locally the A-Darter project is referred to as V3F - you may see it referred to as that in some publications. You will probably find a lot of info about the politics and economic agreements surrounding this missile in the popular press, but again treat any technical info with extreme caution - nothing, and I mean NOTHING, about the technical or predicted technical performance of the missile has been released. Any technical info you will find on the web will be pure speculation.
Finally, to Stimpy 75 - the missile you are refering to with connections to Derby is V4 or more popularly known as R-Darter. It is an AIM 120 class missile currently flying on the Cheetahs. Contrary to what you will find in the press, it is not identical to Derby (unlike V3S which actually IS a Python 3), but rather it was a parallel development of Derby with some early cooperation between Israel / SA, leading to the external similarities in the missiles. V4 is set to be retired with the Cheetah and the plan is to incorporate a more modern BVR missile into SAAF Gripens once the A-Darter project has been completed. If not a locally produced missile, it may be Meteor or something in the same class. Since no final decision has been made about the BVR missile selection for SAAF Gripens, again any such info should be treated as speculation.
Hope that helps a bit, I know it must be a pretty frustrating topic for people with interest in the international defence industry.