I'm not talking about 1964 bumper jacks. Nothing about the '65-and-earlier spring would prevent its use with the type of jack you have. The difference is at the bottom end of the spring. The '66-up spring has the full circular loop for the jack handle to go through. The earlier spring has a U-bend with a horizontal "tail".
It must mean that the change were done in the early/middle of the 1965 production year.
Maybe, but unless some kind of solid documentation comes forth about it, that's likely to remain a guess. Unfortunately this spring doesn't seem to be listed in the parts catalogues at all.
