Hexagon shape. Also, lightly used Stant Racingaluminum radiator cap. Round shape. Both fit standard size radiator necks.
New OEM Replacement Radiator Cap- Stant 10242 (16 PSI)
Stant R29 Radiator Cap 13lb 1965-1979 Mustang Ford Chevy Fiat 124 Porsche NOS
Stant R30 Radiator Cap 16lb 1965-1985 Chevy Ford Corvette Camaro Vanagon NOS
Stant 10362 Radiator Cap
Stant T16R Radiator Cap
Stant T16V Radiator Cap