This is a Dead Air Armament known as the Key-Mo, model DA428. 

If you're one of the many owners of a SilencerCo(R) Omega and have ever longed for a simpler and more reliable mount" your salvation is here. The Key-Mo(TM) also gives your SilencerCo Omega a much-needed upgrade. Simply unscrew your existing mount" and thread the Key-Mo on. Your silencer is now compatible with the Dead Air Key Brake and Flash Hider - giving you the best mounting system in the industry. 

Features: 

4.5 oz. weight 

Fits SilencerCo Omega