Oh, that? It's a sticker meaning that the car passed technical inspection at Sebring that year. VRM sells those separately in their Number Circles & Things sheet (VRM-010).
Hopefully there's still some time because I need to get a replacement windshield for my '66 Imperial and a few more parts for my '63 T-Bird. I guess I waited too long on this one.
The best thing to do is wait a month after the paint dries before clear coating with TS-13. The shinier the surface, the better the decals will stick. That paint job looks great on that car.