If so, then it's looking more and more like the door-or-no-door thing could be more of an inconsistent build thing than it is a factory numerical sequence thing.
I'm wondering too, if it's not in fact more of an inconsistency among DMV's and how they register different vehicles.
If that's the case, that would explain why some "77's" got gas caps even while others already had doors.
We've seen DMV data entry errors here many times before. Wouldn't take much of an error to skew the first few months of a model year into the previous one.
Seems like it would be more of a production error thing, as to which '77's got a single gas door, where others had dual doors even with single tanks.
Sounds like this one has two doors? Do you know if it's got two tanks?
Ford probably thought early on it would continue to do the single/dual tank fenders different from each other right from the get-go, then realized it was a pain and would be better/cheaper to just make one fender for all models, and then just bolt the door shut on the single tanks models.
Paul