Have you looked for the broadcast sheet?
The broadcast sheet was sometimes left in the vehicle as it left the factory and might be found under the back seat, under the front seats, under the carpet or taped to the back of the glove box.
http://1970chargerregistry.com/Production/500_sheet_decoder.html
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I found mine between the back seat cushion and seat springs along with another broadcast sheet for a different car. Also, found a remnant of another broadcast sheet for my car between the front passenger bucket seat cushion and seat springs.
Other owners have purportedly found build sheets in the drivers seat back.