This link below along with some help from forum members might be a good resource.
http://www.statetrooperplates.com/sp-hp.htm
I used this to validate my 1990 North Carolina State Hwy. Patrol license plate. Good luck!
__________________
Mike
1992 Michigan State Police SSP Mustang #5114
1993 Wisconsin State Patrol SSP Mustang
1993 Missouri State Highway Patrol SSP Mustang P641
http://members.fotki.com/MOstang/