I've done a bit of research, and i can't come up with what the deal is with this blue frame. The head is definitely different than any other Broncco i've seen. But it is original, there's no doubt about that. The welds and bends are factory. The only thing I've come up with is Broncco made a design change. Their typical head/downtube/uptube junction was kind of a crappy design. I have another Broncco frame, where it failed exactly there (at the head.) Because the top tubes intersect the head, more or less, in the middle of the neck tube, it allows a bent fork to possibly break the head/top/bottom tube joint. That's exactly what happened on this other Broncco frame i have.
To change the design, they had to put that slight (5 degree?) bend in the top tubes, to bring them up, to intersect the head tube at the top. Now the bottom tubes are at the bottom of the head, and the top tubes are at the top of the head. This gives a much stronger head junction, less likely to break in a front end collision.
Anyway that's the best I have come up with. Though I can't find any pictures of another Broncco frame with this frame change.