Welcome to the forum.
The bad news - that is a Fairlane GT emblem for an automatic transmission equipped car, the part number on the back - C6OB-166098-A - was not used on Mustangs. The "O" between the "C6" and the "B-166098-A" is the key. The 66 Mustang emblem said GT only.
Now it get tricky. The first 65 Mustang GT emblems were ceramic, thru 10/1/65, C5ZZ-16098-D, and said GT. From 10/1/65, they were plastic, C6OZ-166098-B, and were the same as a Fairlane GT with a manual transmission, GT.
Jim