Actually Harry Selby bought the push-feed .458 after he shot the barrel out of his .416 Rigby, a Rigby rifle. He sent the Rigby back to England to be rebarreled and by the time the .416 finally came back, he liked the .458 so much he kept using it. Or at least that is the story that Joe Coogan has told me, more than once. Joe worked as a PH for Harry for many years, and knows him very well. He just saw Harry last year.
Of course part of the deal was that Harry could get a realy good chunk of money for the .416, and did, from a former safari client. I got to shoot it a few years ago, and afterward put together a CZ that pretty much matched it. One of the things about Selby's rifle that is rather surprising is that it's built on a standard--not magnum--98 Mauser action, which is supposed to be real no-no. But it has been working fine for thousands of rounds.
I knew Finn fairly well, and in fact have shot all his African back-up rifles. He very much preferred his old .375 M70 above everything, but had nothing against his push-feed M70 either--though from what he said, it was definitely 3rd in line behind his .375 and his Westley Richards .458.
But I'm not arguing about push vs. controlled, just making some observations.
You are spot on, once Shelby got the 458 push feed he never went back to the 426 Rigby which was built on a Millitary Mauser action.
The debate "push feed VS controlled round feed" is much to do about nothing IMHO
