The bombs produced by the South African program were large, crude and heavy. They were an entirely indigenous design. Anything Israel could have offered would have been smaller, lighter, and better adopted to either tactical or strategic needs. The Israeli denial is far more credible than the allegation that it offered to sell nukes to South Africa.
Two mortar shells explode in Golan Heights, no casualties or damage reported (Haaretz)
from the article: Israel denies offering nuclear weapons to Apartheid South Africa