I have a couple of these radios and the center of the knobs match the cases. They made an orange one, so the knobs are probably replacements as you suggest. Although these are heavy metal knobs , they are prone to pitting. What I suggest is to get some matching spray paint or air brush paint, mask off the outside of the knob and repaint. Model airplane epoxy or urathane paints come in lots of colors, small sizes and generally give great results. Might want to also use primer if surface is pitted or chipped. If you know someone who paints MC helmets, try to get their help.