This is a Leupold 1 Piece Base known as the Standard, model 51736.
The Leupold steel standard bases and rings are the most versatile of any mounting system they offer. The rear ring is secured by windage adjustment screws" allowing the scope's internal adjustments to be reserved for precise sight-in. Additionally" shims can be used for gross elevation adjustment" reserving the scope's elevation adjustment for bullet drop compensation dial applications.
Features:
Forged and machined steel construction
Maximum accessibility