From another Jeep forum (so I deserve no credit):
That is just a production line data control tag. It was attached while the Vehicle Identification plate was being put on at the beginning of the production line when the tub was moved into position to meet the chassis frame when the two meet at the end of the run.
While the tub gets dipped and painted, the chassis frame gets all the drivetrain components attached and so on.... then the completed body meets the frame at the end of the run from above and the two are attached. A line inspector confirms that the body and chassis are a match then uses a hole puncher to mark that tag. That is why that tag is thin and held on with only one pop rivet, it allows the tag to be pulled up and punched. Each assembly line inspector was assigned a letter code that he or she punched for confirmation.