http://www.lowridermagazine.com/tech/1008_lrmp_valuable_vintage_parts/photo_09.html
This page shows a couple of genuine vintage GM caps. You may have to scroll down to find the photo. (click on photo to enlarge)I was given a used one by a friend of mine and the patina of it matches my truck very well. It fits on my '45 GMC and I would think it would fit on most years of GM truck.