It doesn't look rusted just dirty around the numbers. I might be tempted to remove it, soak it in some hot water and work on it with an old toothbrush.
looks like ....
BH27 - Barracuda Convertible
41 - 318 2-bbl engine
5 (395) Automatic transmission
23? - Tires 6.95x14 white wall x5 (spare)
511? - scheduled production date May 11th (1968)
xxxxx - order number
7-8 - wheel opening moldings
AX-1 - 2.76 differential ratio. open
TRM-H6X - High trim grade, 6- bucket seat, x-black color.
PNT-GG1? - Forrest Green metallic color, single stage.
U-B? - Black inner door frame color
R-1 - AM radio
b-4 - Bucket seats
u-1 - ordered car.