I also think the data gets skewed as the years pass. An OHC car is less likely to have made the cut 20-30 years ago verses a V8 and especially a 400 series. It simply went to the wrecking yard. However, I am not sure that was the case the day they were built. Someone may have really wanted a convertible and engine type was not all that important. To save money, they opted for the OHC. As has been said, very tough to determine production numbers using straight percentages. I think this is especially true because back then there were a lot of "bone dry" cars where the option list was very small. Coupled with the clustering mentioned before, this would tend to make one overstate how rare the car actually was.