Axle codes. YB was not applicable to your car. It would have had a 2.78 as mine does, as it is a 400 auto with A/C, unless a special order ratio was selected.
I will try to find my protect-o-plate decode info. Nice 'bird!
If you click my Yellowbird link you'll get a lot of info about your car as well, as they are quite similar beyond the roof.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
