Mystery part fell out while replacing glass digitizer

I was following the instructions in "Replacing Tungsten E screen. PAL5037" (Replacing Tungsten E screen. PAL5037). When I was disconnecting the LCD Flex connector, the buttons and a small black plastic piece about 5/128” diameter x 5/16” long fell out, and I have no idea where the plastic piece belongs. Can you help? I have attached an image in the format of the Step 6 layout.