At the time Testors had just put out some new colors so all I know for sure is that it is Testors custom laquer system. Do not confuse it with Testors Enamel model car paint which will NOT stick to Lexan.
Aside from that Tamiya and Pactra and Spazstix have some great metal flake silver paints.

Your Mustang there is probably Testors Silver.


But you might want to have a look at these.
Sterling Silver Metallic (AMC)


Light Gray Metallic (Chevy) Auto


Or These
Indy Silver

Metal Flake Silver