Thanks Jeff, seems your info is hard to wrap any logic around. I guess it's hard to say if some of the early cars had been messed with.
Understand that is why I mentioned I would have to find a way to illustrate what I was finding at San Jose in 65
Looks like the flat with screw version is somewhat defined
Yes its hard to prove that the bumpers are original to each car. Choose only unrestored cars for the samples and since there were no reproductions available for much of the past and NOS ones often were not always right I think
1- there is a possibility that they used more than one vendor at San Jose and they sourced from both for a fairly long period that first year.
2- If it was just a result (the lack of a hard stop and start to the second and third version) of owners changing them the ratio of changed verses not changed would have continues though the whole year and we're not seeing that so IMHO that suggests that the first possibility is more logical at this point.