Most likely would be the same as the 375H&H insert or what Blaser calls Magazine 8. They are near mirror images of each other, no surprise as they were introduced just 13 years apart and from the same gun house.
300H&H
375H&H
Magazine 9 covers 257WBY, 270WBY, 300WBY, 300WM, 338WM, 7RM...