Here is something I found online by searching for ... "valve covers "Thunderbird" lettering".



M-Code Thunderbird High Performance 390 Sports V-8 Engine


These limited edition engines differed in many details from the standard Thunderbird 390 Special V-8, and are identified by the "M" code letter stamping located in the fifth space of the Vehicle Identification Number (VIN) as well as on the data plate mounted on the driver's door lock facing. We will add to this list as we verify information.

Chrome Dress-Up Kit (Standard with Sports V-8 option; Ford part numbers appear in parenthesis)
- "THUNDERBIRD" raised lettering on valve covers (RH: C3AZ-6582-F; LH: C3AZ-6582-G)
- Valve cover retaining bolts
- Oil breather cap (C2SZ-6766-A)
- Fuel log (C2SE-9D303-A)
- Dip stick loop handle (C3AZ-6350-C)
- Power steering reservoir cover (C2SZ-3A582-A)
- Power steering reservoir filler cap (C2SZ-3A006-A)
- Brake master cylinder reservoir cover (C2SZ-21162-A)
- Radiator surge tank (C2SZ-8A080-A)
- Radiator cap (C2SZ-8100-A)
- Finned aluminum oval air cleaner assembly with Thunderbird ornament (Air cleaner: C2SZ-9600-A; ornament: C1SB-63606A08-A)