There is a bit of a story behind that "NOT FOR REVOLVERS" stamping on the .455 Automatic ammo pack. The .455 Auto is semi-rimmed, and so will fit and fire in .455 Webley revolvers. But the auto pistol round is in about the same pressure category as the .45 ACP (c. 21k psi), much too powerful for revolvers designed for a cartridge generating only c. 12k psi).
The flip side of that coin is that after WWII, many surplus .455 revolvers were sold in the U.S. reworked to take the .45 ACP with half-moon clips. Those should never be fired with full power .45 ACP ammo, which has been known to blow cylinders on the old guns.
Just for info, and NOT recommended, but the Webley auto pistols can be fired with .45 ACP by seating the bullet deeper and feeding from the magazine so that the extractor will catch the rim of the shorter case.