This is a high quality, aluminum muzzle brake compensator.  It is made in the USA (Minnesota actually).
 
Features include:
  • 9mm and .380 caliber
  • threaded for M13.5x1mm thread pitch left handed (LH) threads (most common 9mm threads internationally)
  • COUNTERBORED: this feature on the slide facing side lets the brake get closer to the slide to make the gap smaller!  (see picture of the side view)
  • width is sized be smaller than Glock 17, 19, and 26 (0.85" wide - measure yours to see if it will match) so it goes in and our of holster easily
  • flush with the top of the glock slide
  • distance (or gap) from the front of the slide depends on your particular barrel but is reduced by up to 0.240" because of the Counterbore.
  • add roughly 0.84" to the length of the barrel (however far out yours sticks)
  • Made of 6061 Aluminum
  • Anodized black hard coat type III (Select Color in Drop down).
  • held on with #8-32 x 1/8" set screws (included)
  • tighten set screws to 10-12 inch pounds
  • Loctite Included
  • Allen or hex wrench NOT included
This reduces felt recoil for competition, or duty.  Works better with higher grain/higher power loads (they have more gas to push back against the brake with).  In some cases, very light gamer loads will not cycle the action unless springs are changed out. 

Set screws are tightened with a 5/64" allen wrench NOT included.  When tightened down the tops of the allen screws are below flush with the side of the compensator.  On screws this small about 10 inch pounds is all the tighter you want to get them.