A Ford 13# chrome cap is C9ZZ-8100-A (Motorcraft RS 512) now obsolete from Ford but still around at Mustang vendors, etc. Like here:
1965-74 Ford Mustang Torino Shelby c9zz-8100-a radiator cap (no endorsement, just a sample)
The actual cap used at Shelby is likely a
zinc plated unit like this:
Mustang Radiator Cap Smco Zinc 1964-1966 | CJ Pony Parts