I don't think it's a seal, but there is a piece of embossed sheet metal in the bottom of the bearing bore.
I think it's just a spacer. You could probably fabricate one from 1/16" brass using circle cutters and small files from Lowes or home depot.
Kirk is right, Its called a rear grease seal, Thompson's garage sells em for 4 bucks. from the outside in its bearing, grease seal, c clip. if you ever want to get em out, the easiest way to do it without damaging anything is to drop the hub with the drum attached from waist height onto a wood board or wood board with cardboard over it, drop it so it lands flat on the part of the drum that touches the backing plate. takes a couple of lite drops but the momentum causes the bearing to push the seal out, the c clip must be removed before you do this.