Lyman 12S Slot Blank. 375in dovetail mount. These Rifle Parts by Lyman fill the empty dovetail cuts on your firearm. The Lyman #12 Slot Blank is useful for when the rear sight is removed from a barrel since the factory rear sight may be visible in your sight picture after you've installed a receiver sight. For a quality part to assist with your optic setup get the Lyman 12S Slot Blank Piece. Specifications for Lyman 12S Slot Blank: Manufacturer: Lyman Color: Black Package Contents: Lyman 12S Slot Blank Lyman 12S Slot Blank 3121040