Nice photos, seems like SAC was on a goodwill mission.
B-50s deplyed to the UK quite a bit, but South Africa is a bit far for that, so it was either a goodwill mission or a long range exercise.
It could be the 40th BW...its predessor unit (the 40th BG) used the markings on its B-29s in WWII. Of course, it could have been reallocated in the post war reorganizations.
It also
could be the 28th BW. At least they had the "Triangle S" on their B-36s in the 1950s.
But they went from using B-29s to B-36s...I don't think they ever used B-50s.
I'm sure someone here will have the facts.
If you'll allow me a small correction."Washington" was the name given by the RAF to 88 B-29s loaned to the RAF between 1950 and 1958 as Lincoln replacements.
B-50s were a different aircraft (very simply put...a reengineered and reengined new-build B-29 variant). It shared the "Superfortress" nickname with B-29s. None were used by any other air force.