When I took the trigger assembly off this piece of metal(see photos) fell out on the table. What a way to start refurbishing an old 99M1 rifle!! Can someone tell what part this piece is off of? Looking down into the receiver I see no jagged part and the parts look clean, no rust, etc. I guess if I can learn to dissemble the receiver housing the broken part will reveal itself, but without some instructions I am a bit overwhelmed. Never attempted such before. Boy those screw heads are small! Any guidance is much appreciated!