Oooh! Oooh! I know this one!

You need a three digit LH flight number FRA to JNB. Say flight 572 for example. That is on LH metal and you will get miles (I did.) Avoid the four digit LH flight numbers, those are on SAA metal. This is a pretty good rule for all airlines in my experience. The flight will have three or four or five flight numbers, one for each code share, and whoever has the three-digit flight number, that's whose metal it is. The rest are along for the ride. Of course it only matters if not all the metal-owners are in the alliance, and that's the FRA-JNB story.