I also have one of these knobs on my truck. Spotted an original one on e-bay a while back. I cleaned it up and repainted the markings. It came out nice. I like it. It seems that they were factory on the 60's trucks but I dont know where I got that idea.
Jeff

"Do not walk behind me, for I may not lead. Do not walk ahead of me, for I may not follow. Do not walk beside me either. Just leave me alone, you're starting to freak me out."

1957 GMC 150