This is a Trijicon Adapter, model TA51.
Fits 3.5x35" 4x32" 5.5x50 ACOG" 1x42 Reflex (with ACOG bases) and 1-6x24 VCOG. Mounts optic to picatinny rail with two thumbscrews.
Features:
Height of Mount to Optical Axis: 3.5x35 4x32 and 5.5x50 ACOG = 1.535 in. 1x42 Reflex (with ACOG Bases) = 1.506 in. 1-6x24 VCOG = 1.535 in.