Left over part after disassembling phone for digitizer replacement

Hi,

Having disassemble my iPhone 4, in order to replace the digitizer/screen assembly, one part fell out and I am unsure where it should go!

Everything else was meticulously removed and documented, but this piece dropped out, along with a small screw.

I thought it came from the corner section underneath where the vibrator goes, but I'm not certain and I can't find any diagrams or videos showing it.

Can anyone help and even point me to some clear documentation with pictures?

I have attached a link showing 2 pictures: one image from above with the iPhone shell next to it for scale and the other showing a side on view, where you can see a slight lip.

I'm now wondering if this is a snapped-off part of the original screen, although in that case, I don't know where the screw came from...

http://1drv.ms/VBldCq

Thanks in advance for any advice!