Door data tags, also called Vehicle Certification labels, are located on the inside face of the driver's door. These offer really good information on some of the options a particular car was originally equipped with, and can also aid in the identification of a Special Service Mustang. Here is a guide from Ford literature showing how the labels are decoded, followed by pictures of actual Special Service Mustang tags:

Image courtesy of the 1984 Ford Car Specifications book.

1983 Colorado State Patrol Door Data Sticker, DSO 76-0029.

1985 California Highway Patrol Door Data Sticker, DSO 72-0406.

1987 Texas Department of Public Safety Door Data Sticker. DSO 57-0119. Matt Buckmaster photo

1988 FBI Door Data Sticker, DSO 83-0077.

1991 FBI Door Data Sticker, DSO 83-0191.

1991 Santa Clara County, CA Door Data Sticker, DSO 72-0723. Bret Traywick photo.

1992 Gainesville, Florida Door Data Sticker, DSO 24-0247.

1993 Florida Highway Patrol Door Data Sticker