John, here are the pictures that I have of the original 15" wheels that I took off of my '66 C-10. They are 6 lug, and had three metal tabs/ears formed into the ring around the center. I know '63-'70 used the same rims, I'm not sure about the '60-'62 trucks though.

http://rides.webshots.com/photo/2233668130100720268CJOWhy
http://rides.webshots.com/photo/2616623150100720268qSfAQF