More food for thought...
Refer to the picture of the Mustang on its side. This whole area, known as "the swamp", has cars with license plates from 1971 to 1973 only. They are stacked close, and there are trees all over. The only way to get in this area is to walk. I conclude that this Mustang was put in its current resting place no later than 1973, at ~6 years old. Thus there would be no reason for someone to apply early 80's incorrect replacement stickers.
This car also has a spare tire sticker with a C7 part number, also included here for reference.
I must conclude that although the majority of original safety act stickers include a C8 part number, that some vehicles made it through the line with this alternative. The VIN is 8F01C184867.
Are there any other explanations that I might have missed? Thanks, John