It certainly wasn't the SP. (I don't recall them ever on the LHR route.) The time frame for the Upington stop was just before I left SAA so around 1976 - 1978 so IIRC the 74 Classics had been upgraded to Super B by then. (don't think the Upington tech stop lasted too long as I remember sitting on the ground waiting for the temp to drop to get the last drop of fuel in before setting off north).
1978, BA 747-100 - flew LHR to JNB via Nairobi on the way there and Zurich on the way back. First time up to the flight deck in-flight, remember like it was yesterday. Just don't ask me what I had for dinner last night.