The piece you are referring to is a hammer safety. I have a fullstocked version of this same 20 gauge rifle. Husqvarna No. 4. If you look closely, there is an extension behind the hammer that keeps it from being cocked. Press down on safety, and it moves forward while cocking hammer. I guess it is to keep a snag from pulling hammer back far enough that its release would ignite primer. Started showing up around 1902 on rolling blocks in Sweden.