They were attached to the vehicle as it came down the assembly line to help workers know what items to install. They also had a "build sheet" to go by that had color indicators to know what spring (for example) that particular car was supposed to get. In the Mustang world for example, a Boss 302 build sheet might call for a spring with a purple, purple, green paint stripe on it , whereas a six cylinder car might get a org, blue, blue front spring. These are just made up examples but hopefully you see what I'm trying to say. So sometimes in restoring a Mustang someone gets lucky and finds the build sheet stuck up under the dash or laying under the carpet because the line worker would just toss it into the car before the carpet was installed. Back to buck tags, they were often attached to the radiator support so over the years when a previous owner was tinkering under the hood they would remove them and they got tossed. Now that these cars (and Broncos) are decades old it's a great item to find especially if the owner is trying to do a restoration. Things like the buck tag, build sheet, original owners manual and Marti Report are great items to have and display especially with stock or original type cars/trucks.