Unfortunately, what it was originally may, or may not help you with selecting the correct rotor for your setup.
What lug-pattern is it running? What calipers? Looks like a pre-88(or whenever?) Ford axle that's been shortened and converted to a smaller brake setup. Judging just by the caliper pins, I'd say it's likely to be a GM brake setup.
If the rotors are 5 on 5.5, it could be from a Ford full-size Bronco/F150. If 6-lug, then any GMC 1/2 ton/Blazer from 68 to 91, or even later. Not sure if Ford used that type of sliding-pin caliper in later years, but at least up to '79 the calipers slid directly on the mounts.
Good luck. Lot of good info in that Billavista article. Hopefully you can find what you need there.
If the outers have been converted though, it might be hard to glean just what's going to be needed.