This is a Magpul Industries Magwell known as the Hunter 700L, model MAG569-BLK.
The Magpul Bolt Action Magazine Well 700L Magnum is a drop-in detachable box magazine solution for our Hunter 700L Stock. By simply removing your OEM bottom metal from the Hunter 700L" users can add this reinforced polymer trigger guard and magazine well without the need for custom inletting. Designed to work with AICS-pattern long action magazines" this kit also includes one Magpul PMAG(R) 5 AC(TM) L" Magnum for .300WM based family of cartridges. Made in the USA.
Features:
Fits the Hunter 700L Stock and long action .300WM caliber AICS pattern magazines
Includes one PMAG 5 AC L, Magnum magazine
Easily installed