Element seal.
This keeps the two metal parts 18 and 21 from rubbing each other, as well as seal the element compartment the two parts form when assembled. The foam actually fits inside #18.
It is a U shaped formed rubber part. You'll need two if original. They expand something fierce during use, and are quite difficult to get back on and reassemble properly. If left to dry out they shrink back to proper size. So, it's one in use and one hung out to dry between service intervals. The one in the pics is dry. It might be a tad smaller than when fitted onto the edge of #18.