My fault, sorry, Marcus.
I have been working on and for the forum since 9 in the morning (it's an hour to midnight now).
It is a screw but entering from the inside and holding the sear in place; it is not broken off and it does not have a safety function. Each conventional matchlock must have this screw, the earlier ones (like the two I presented here) show rivets in this place.
Again my matchlock chronology 1520-1720 should help settle all questions regarding that earliest and simplest of all igniting mechanisms.
m