it's probably the where the camera mounts. without pictures, i can't know for sure. i recommend following the guide to replace the lcd screen for the phone. it'll show step by step instructions and you'll eventually come across that part in the walk-through.
iPhone 4S Screen Replacement
Step 16, enlarge the picture where the person has the camera in his finger tips. Look where the camera was and you'll notice the piece you're missing. It's normally underneath the black sticker on your logic board where the camera sits. The location of the black rubber piece is a problem I've been having also. I'm almost certain it was on the logic board by the display ribbon connector, the top edge, but i'm not certain. I'll let you know if I figure it out. Please do the same if you find out.
Step 23 also shows the gold clip as well. That screw he takes out holds that gold clip in.
Step 25 also shows a very clear picture. Enlarge the picture to the max and you'll see it at the bottom right.
lol, step 26 pretty much sums up everything...