Let me try to answer this a different way. Your truck could have a 1 inch or 1.5 inch rotor in front and a 2" or 2.5" wide brake shoe in back. You say you've had the truck 9 months? Any of the previous owners might have made a change mix matching your set up. Someone else with a stepside may have a different braking system than yours. I've had at least 100 square body trucks and have only seen the RPO code list on 1989-91. The rest is basically the short options list like the following pictured.