Entirely possible there was a running change. It would likely have been "use old stock until exhausted". The improvement of the later spring with the full coil at the bottom for the jack handle is that it couldn't slip off, which I believe was more than a possibility with the style in the pictures.
If the spring is even listed in the parts catalogue, it would be interesting to (a) see if the same part number shows up in '65 as in '66, and (b) if the numbers are different, if the '65 number supersedes to the '66 number.
I think it's pretty safe to assume that Chrysler wouldn't have cared other than knowing that the later style was an improvement over the original release.