470R, surely you will have noticed that all those cartridges you listed as 'Magnum Express', viz. .350 Rigby Magnum, .425 Westley Richards Magnum, .256 Gibbs Magnum, Jeffery's .303 Magnum, are all bottle-neck cartridges resembling the magnum champagne bottle as per the original definition. It looks more like Rigby's later definition may be the one which has deviated from the original by invoking a minimum bullet weight. I chuckle at the thought of the .577 Snider Magnum! Perhaps Rigby's response to Stonehenge was quoted out-of-context by the press (wouldn't be the first time!), or perhaps the bottle-shape was one of the "other conditions" required to be fulfilled.

In any case, I can't see how describing bottle-necked high-velocity cartridges as 'Magnum Express' is somehow deviating from the original definition.

If you can think of a straight-walled case properly referred to as a 'Magnum' by the Brits, please chalk it up. Otherwise, we must continue to regard this as another American corruption when applied to the .458.