1966 Chevelles have 2 main tags for identification. The first is on the driver's side door near the door hinges. The tag is rivoted to the cowl and gives the main serial number for the Chevelle. A true 1966 Chevelle SS would start with 138*********. If the car is a clone SS, it might read 136*********.
Then there is the second tag which is black and is rivoted onto the firewall. It is on the driver's side of the firewall just to the left of the windshield washer pump. It contains lots of codes for the Chevelle. Things like original paint color, assembly plant where the Chevelle was manufactured, etc. etc.
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dtm