Interior colour code

The five in b5 is medium.

The numbers after the paint / interior codes tell light to dark shades.

1 is the lightest & 9 is the darkest, so 5 is about the middle.

Interior color swatches for 1971 Dusters here;

http://www.hamtramck-historical.com/dealerships/1971ColorAndTrimPlymouth-05.shtml