I think your findings may be tainted. I am not skeptical that you could come up with both styles in 67 . Unless i am missing something It could just as easily be explained that the examples you found of the ball end style on a 67 car had been replaced instead of being untouched? I have changed my point of view before in light of new evidence . I myself would need more evidence before I change a point of view that I have had for a long time because of personal observations. That is just me .Others my think differently.
Bob, all I really did was go through pictures of what are, to the best of my knowledge, unrestored cars. As you stated, unrestored does not mean untouched.
I'm not convinced either way, but I would say that at this point I'm intrigued.
Attached is a chart based representation of what I found thus far. Please note that the timeline represented is not perfect. All I have is the VIN, not the build dates. A Dearborn 124 car may have been built before a San Jose 120 car for example.
Some observations from the chart:
1. Dearborn has only one, so it would be easy to write that off as a post. However, I did inspect this car in person and it was very original, in very rough shape, and not even a repaint in the door jamb areas. The door tag was in place, matched the inner fender, and was never removed.
2. Metuchen has none. But I only found 4 cars with clear enough pictures of that area of the interior to report on.
3. San Jose has 28% ball style, and all except one are "first half". But again the sampling is small.
If the argument is that a lot can happen in 50 years, I would think that a similar percentage would be found across the board. That is what is really interesting.
It would be interesting to see if there are any 1966 date coded ball style mirrors. It would also be great to get more data.
Personally I have it easy - my car had the tapered style originally and that is what is going back on. If I were restoring a 67 that did not have the date coded ball style, I would go with tapered. To avoid the drama, as you often recommend.