This is a Trijicon Adapter known as the ACOG, model TA60.
Fits the 1.5x16S (M16 base)" 1.5x24" 2x20" 3x24" and 3x30 ACOG models. This thumbscrew flattop mount clamps onto 1913 Picatinny rails.
Features:
Height of Mount Surface to Optical Axis: 1.5x16S (w/ M16 base) = 1.660 in. 1.5x24 = 1.660 in. 2x20 = 1.660 in. 3x24 = 1.660 in. 3x30 = 1.660 in.