The number 28 is the paint code for the lower part of the car. Just to the right of it would be another number for a metal top closed car, or a letter for the color of a vinyl top or a convertible top.
On your data plate the 28 is there, and then there is something that is stamped, but cut off, to the right of it. It's possible your dark blue car had a blue top originally, and that would be depicted in the letter we can't see.
So that's what I meant by saying it's too bad that code had been cut off, probably the plate itself wasn't lined up properly on the machine that did the stamping.
1970 Oldsmobile Cutlass
1970 Oldsmobile Cutlass Supreme convertible
1970 Oldsmobile Vista Cruiser
1965 Oldsmobile F85 parts car