This is a Yankee Hill Machine Co Muzzle Brake known as the Slant Brake/Compensator chambered in 9MM, model YHM-85-MB-A.
This 9mm Muzzle Brake and Compensator hybrid is designed to reduce recoil as well as muzzle rise for a quicker follow up shot.
Features:
1/2X36 thread pitch
Includes crush washer Melonite QPQ black finish