NPD does not have it; I was unable to find any reference to it. The part is an "ornament - quarter panel - rear" with basic part number 290A76. The "gasket" was not serviced (not in the 67, 68, or 75 MPCs) and I would speculate was most likely attached at the plant where the part was cast. I ended up using 3M body caulk to seal these and provide a small gap to prevent direct contact to the painted body surface.
67 289 GTA Dec 20 1966 San Jose