I agree, the story was that Wally was hunting along a river, and an "arroyo" (don't know what the Swahili word would be) branched off. Wally asked a native woman washing clothes if she saw a wounded animal go up the arroyo, and she said no.
One of his assistants was trailing behind with the 9.3x62, and saw movement in the arroyo. He went to check it out and it turned out to be the wounded buff. IIRC Wally came running back when the assistant put 5 rounds into the chest of the buff.
It wasn't recorded what bullets were used, nor where Wally shot the buff that dropped it (although a headshot under the boss would probably be my guess).
Of course Capstick told of a 470 Nitro Express solid gouging out a hole in the boss of a charging buff, which is why he let go with both barrels and was thankful the other bullet did the job. Although he also recalls a German lady who cleanly took buff with a 7x57 with side shots to the boilermaker.
So that is why the 9.3x62 is not a "stopper" cartridge even though it is legal to hunt dangerous game with "by experienced hunters" according to some laws in Africa. It is a good round, fully adequate to take game. I wouldn't consider the 375 H&H a "stopper" cartridge either, as it is the minimum bore in many countries that don't have an exception for the 9.3x62.
It should be noted that Capstick served as a game control officer, which gave him a solid opinion that 'bigger is better' when facing down dangerous game (except for leopard, he used a 12 guage for leopard, with #1 buck).