From Unintended Consequences by John Ross.
".450 Ashurst," Kearns said immediately. "Full length .375 H&H case necked up to .458 caliber and blown
out straight. Holds more powder than the .458 Winchester magnum, but only if you've got an action with a
long magazine.
Google produces pretty much nothing -- one thread on Cast Boolits confirming that John Ross didn't just make the name "Ashurst" up like authors sometimes do.
Jack Ashurst lived in Grangeville, Idaho, and took over Lyman McCrea's gunshop and cartridge line about 1941. He dropped the McCrea cartridge designs for his own. He had four .25 calibers, based on the .22 Savage HP, the .30-30, the .30-06 and the .275 H&H Magnum cases, and a .270, .280, 7MM and .300 cartridge series all based on the .300 H&H Magnum case. The cases featured the short necks and abrupt shoulders typical of wildcats of the period, although several of them retained the body taper of the parent cases. The usual blistering velocities were claimed for these creations. This from Wildcat Cartridges by Richard F. Simmons. I think P.O. Ackley described some Ashurst cartridges in his Handbook for Shooters and Reloaders. However, neither list a 6mm Ashurst.
The parent brass. 2.85" long.
The 458 WinMag (1956) case is 2.5" long. COL = 3.34".
The 458 Lott (1971) is a 357 H&H case blown out with a slight taper, trimmed to 2.8". Because of the taper, bullet tension in the neck is going to be an issue. Some manufacturers reload the 458 Lott with a ghost shoulder to solve this problem. a 458 Lott rifle can shoot 458 WinMag ammo. COL = 3.6".
The 450 Ackley (1960) solves the taper problem. It's a 375 H&H case blown out straight all the way to a sharp shoulder and short neck, with an overall length of 2.85". A rifle in this caliber won't work with 458 WinMag ammo.
The 450 Watts (pre? 1950) (sometimes referred to as the 458 Watts) is also a 375 H&H case blown out with a slight taper, with an overall length of 2.85". While the brass is longer, the cartridge overall length is the same (on paper) as the 458 Lott -- the bullet is seated deeper (with the right rifle and a long long action, this can of course be handloaded differently). Of course it comes with the same bullet tension issues of the Lott.
So the 450 Ashurst is the 450 Watts, looks like. Well, from that description.
But, in Unintended Consequences the rifle in question is an Enfield "...ears on the receiver bridge had been milled off..." P-14 or M1917, presumably. The P-14 magazine well is 3.38" inches long. This makes the 450 Ashurst pretty damn close to the... 458 WinMag. Assuming the action had not been stretched
Later in the book, Lott mentions a 450 Ashurst on a BRNO ZKK 602 action. COL for this action is at least 3.6", so let's go with my first guess.
Maybe I'm overthinking things.
There's also the 458 Express, which at 2.99" is even longer.