Rubber Gromet left over after strip down and rebuild

I recently pulled my iphone apart and followed the tutorial, half way through the rebuild I came across a rubber boot/ grommet about 8mm x 2mm x 2mm, with a cut out running long ways down the centre of it. I have browsed the photos in the tutorials to see if I can find where it belongs but have had no luck, looks as if it may insulate or fill in a space somewhere, with the cut out sitting on a face or part. I would like to have no spare parts after the rebuild, if someone can help that would be great.