This replacement part is a factory original from Magnum Research. Factory replacement parts are manufactured to the exact same specifications and tolerances and use the same manufacturing materials as the original parts which guarantees excellent fit and reliable operation.
The 50AE (Action Express) is also known as the Mark XIX
The 44 Remington Magnum are known as the Mark VII.
The 50AE (Action Express) can be converted to 44 Remington Magnum or 357 Magnum simply by changing out the slides.
Keeping a few extra factory magazines around is never a bad idea; extra magazines save loading time at the range and also serve as a backup for those days when you simply cannot remember where you put your primary magazine.