These often missing parts are glued into the two recessed areas designed to hold the female portion of your lap belt. They prevent wear on your buckles and keep them from rattling if you are one to leave the shoulder harness off. Sold individually, 2 required per car. There are at least two different styles of these, one version has a ridge near the top. Either one may be shipped.
Ford part number C7ZZ-65044E16-A.