Join Date: Sep 2008
Location: Bavaria, Germany - the center of 15th and 16th century gunmaking
It is as Fernando K said:
There is a small, thin integral hook at the underside of the pan cover which reaches into a longitudinal slot at the inside of the wheel. When the wheel starts spinning back it pushes the pan cover open, which otherwise is kept on the pan by the pressure of a long, thin, one-armed spring.
I tried to mark the arm of the pan cover red on the two bottom images.