At the end of the day, the only thing that matters to a DOT is what is on the title/registration. If it says 1978, it is a 78 car that is exempt. If it says 1979 (even though built in Dec 1978), it is a 79 car and needs to be tested. If your registration says 1978, then case closed.

Bert

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'70 111 280SE/c 3.5 (4 spd manual) - sold
'63 MGB
'73 MGBGT V8