This is a Wheeler Tool known as the Delta Series, model 156211.
The Delta Series(R) AR 308 Mag well Block is designed for use with the Tipton(R) Best Gun Vise(TM). The vice block locks into the magazine well and provides a steady base for cleaning or light gunsmithing on your LR-308 Rifle.
Features:
Can be mounted directly to your bench top with the included fasteners
Locks into place using magazine lock on your receiver
Built-in hammer stop prevents damage to lower receiver
