that's your sear engagement surface, the lower part in that window is the trigger side, while the upper part is part of the assembly that holds the bolt in place.
when the bolt is cocked the action of holding the piston back under spring pressure should push the upper part down until it's stopped by the lower part, pulling the trigger moves the lower part out of the way allowing it to drop firing the gun.
the reason there's a window there is so you can visibly see when adjusting the trigger pull what's going on, eg how long or short a pull is required to trip the sear.
it's possible to have the overlap set so light the mildest jolt could make it trip, but it looks more than enough there.
i suspect your problem is more on the upper side, where the upper assembly has a hinged arm that catches on the back end of the piston.