Been through it twice, once with my Cobra and then with my GT40. Liked to scare my wife to death when a fellow in a suit and badge showed up at the door and wanted to see the GT40.
Anyway, he was a DOT inspector that came by to check that the car wasn't part of a chop shop deal. After checking it out and was satisfied with what he saw, he said I'd get a VIN with an "official" number in the mail that I was to place on the car. ERA's plate with number didn't satisfy him. i guess what triggered the inspection is I had the car registered as a '65 Ford. The Cobra was registered with a certificate of origin from ERA.
They are getting tougher, so except an onsite visit from
the man if you built you car and are or have registered it.