I found the same thing yesterday servicing my Poulan/ Craftsman 358357181. the black plastic web is an anchor that keeps the gas cap attached while unscrewed. The clip--which looks like a multi-pointed star--accepts the stem of the black plastic web and clips into the inside of the gas cap. I was able to snap the star back into the center ring in the under-side of the gas cap.
bentonsedge, January 2011