it's lost lol. try find it in the box, if it don't have in the box, you got bad luck. take the screw right next to in hop-up unit and go to hardware store find one. I'm not sure much because it's very tiny .
If you still have cash, updrage hop-up unit because the stock is bad, easy broken.
If you still have cash, updrage hop-up unit because the stock is bad, easy broken.