This is a Leupold 1 Piece Base known as the Dual Dovetail, model 54218.
Leupold's steel Dual Dovetail bases and rings are one of their strongest mount systems" designed for large caliber rifles with high recoils. The twist-and-lock action of the rings ensures a strong connection with the one or two-piece base.
Features:
Forged and machined steel construction
Maximum accessibility
