The 60-68 Ford Car Parts Manual (aka MPC) shows a transition date from ceramic GT emblems, C5ZZ-16098-D to plastic GT emblems, C6OZ-16098-B of 10/01/65. Note, the ceramic was a Mustang service part, the plastic is shown as a Fairlane service part. (Fairlane application was a 66 390 with a manual transmission only.)
Someone is going to have to pull a known original one off their car and read the numbers on the back.
Jim