I field stripped the mustang to clean. While I was brushing out the lint and dust along the rails of the frame a small arched metal piece sprang up near rear of the slide. I pushed it down and now it won't go back to the same spot that I first noticed it(I didn't force it ever). This part appears to be pinned at its base near the safety lever on the left side or the frame. It is swinging up and down and prevents me from fully inserting the magazine. Is anyone familiar with this problem and if not, does anyone know where I could look at a diagram to figure this one out?