Other companies make them too. Check out some NRA highpower sites. Many people running "spaceguns" use them. I have one, didn't like how it filled/crowded the trigger window so I haven't used it in years. It allows you to drop the bolt with your trigger finger. No more reaching over the top or racking the charging handle to close the bolt. I can't remember who made it. I think I got it at Sinclair International or Creedmore Sports. It simply slips over the original bolt catch and is held in place by a set screw. Some have reported sluggish bolt catch or even failure to hold open on an empty magazine, though this never happened to me it seems possible.