It's a rebuild, "show me the car fax"
Notice the exterior color is not on the engine bay firewall, and yes, you are correct, Ford did not install diamond plate in the bed.
Interesting to see if the bed superstructure still has the rotted bed-wood under the diamond plate...
Could be a "pig with lip-stick"
1986 F-150 Flareside 4x4, 351, 4-v, ZF5 speed. AC, Cruise, Tilt, Slider, Digital clock, Radio, Lariat seat, Pwr doors/locks