A66 cars from both plants do have a registry;

http://www.hamtramck-historical.com/register.shtml

The earliest ones I have fender tag data for from both plants so far;

Hamtramck - 922

LA - A03

The page at this link;

http://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1970/70_Challenger0024.jpg

shows the A66 code in the data book. I have another page with a list of revised pages to 'change out' to update the book, sadly, it is not dated. This page is on the list though. If anyone has page 116 & it looks different than that one in any way, I would love to see it.

Barry