I just found another NOS early 1973 Firestone 500 so now I have three and need one more for the display set. (Not counting the trunk mounted spare that has never been bolted on the car).
I found this one on a Corvette forum. They guy had it for the past 20+ years. This size (GR70x15) was the same size used on the Corvettes in the 1970's.
