This is the Daniel Defense One O’clock offset rail mount for Picatinny rail systems.
This Rock & Lock system is optimized for mounting miniature red dot optics in the 1 O’clock (right-hand) or 11 O’clock (left-hand) positions for quick, non-magnified, transitions when employing magnified primary optics on AR-type rifles with MIL-STD-1913 rails.
The geometry allows for zero cheek-weld shift during the transition. This facilitates very fast transitions from magnified to non-magnified optics, and back.
The 1 O’clock Offset Rail can also be used to mount a variety of lights or other accessories that mount to a MIL-STD-1913 rail. By flipping the part upside-down, one can mount a light in a good position for use with a vertical foregrip.